ESQL: Skip unsupported grapheme cluster test (elastic#115258)

This skips the test for reversing grapheme clusters if the node doesn't
support reversing grapheme clusters. Nodes that are using a jdk before
20 won't support reversing grapheme clusters because they don't have
https://bugs.openjdk.org/browse/JDK-8292387

This reworks `EsqlCapabilities` so we can easilly register it only if
we're on jdk 20:
```
FN_REVERSE_GRAPHEME_CLUSTERS(Runtime.version().feature() < 20),
```

Closes elastic#114537
Closes elastic#114535
Closes elastic#114536
Closes elastic#114558
Closes elastic#114559
Closes elastic#114560

Author: nik9000

docs/reference/esql/functions/description/reverse.asciidoc

@@ -1236,6 +1236,7 @@ off_on_holiday:keyword | back_home_again:keyword
 
 reverseGraphemeClusters
 required_capability: fn_reverse
+required_capability: fn_reverse_grapheme_clusters
 ROW message = "áéíóúàèìòùâêîôû😊👍🏽🎉💖कंठाी" | EVAL message_reversed = REVERSE(message);
 
 message:keyword                | message_reversed:keyword

docs/reference/esql/functions/kibana/definition/reverse.json

@@ -32,6 +32,12 @@ public enum Cap {
          */
         FN_REVERSE,
 
+        /**
+         * Support for reversing whole grapheme clusters. This is not supported
+         * on JDK versions less than 20.
+         */
+        FN_REVERSE_GRAPHEME_CLUSTERS(Runtime.version().feature() < 20),
+
         /**
          * Support for function {@code CBRT}. Done in #108574.
          */
@@ -133,7 +139,7 @@ public enum Cap {
          * - fixed variable shadowing
          * - fixed Join.references(), requiring breaking change to Join serialization
          */
-        LOOKUP_V4(true),
+        LOOKUP_V4(Build.current().isSnapshot()),
 
         /**
          * Support for requesting the "REPEAT" command.
@@ -279,7 +285,7 @@ public enum Cap {
         /**
          * Support for match operator
          */
-        MATCH_OPERATOR(true),
+        MATCH_OPERATOR(Build.current().isSnapshot()),
 
         /**
          * Removing support for the {@code META} keyword.
@@ -349,7 +355,7 @@ public enum Cap {
         /**
          * Supported the text categorization function "CATEGORIZE".
          */
-        CATEGORIZE(true),
+        CATEGORIZE(Build.current().isSnapshot()),
 
         /**
          * QSTR function
@@ -375,7 +381,7 @@ public enum Cap {
         /**
          * Support named parameters for field names.
          */
-        NAMED_PARAMETER_FOR_FIELD_AND_FUNCTION_NAMES(true),
+        NAMED_PARAMETER_FOR_FIELD_AND_FUNCTION_NAMES(Build.current().isSnapshot()),
 
         /**
          * Fix sorting not allowed on _source and counters.
@@ -387,45 +393,40 @@ public enum Cap {
          */
         PER_AGG_FILTERING;
 
-        private final boolean snapshotOnly;
-        private final FeatureFlag featureFlag;
+        private final boolean enabled;
 
         Cap() {
-            this(false, null);
+            this.enabled = true;
         };
 
-        Cap(boolean snapshotOnly) {
-            this(snapshotOnly, null);
+        Cap(boolean enabled) {
+            this.enabled = enabled;
         };
 
         Cap(FeatureFlag featureFlag) {
-            this(false, featureFlag);
-        }
-
-        Cap(boolean snapshotOnly, FeatureFlag featureFlag) {
-            assert featureFlag == null || snapshotOnly == false;
-            this.snapshotOnly = snapshotOnly;
-            this.featureFlag = featureFlag;
+            this.enabled = featureFlag.isEnabled();
         }
 
         public boolean isEnabled() {
-            if (featureFlag == null) {
-                return Build.current().isSnapshot() || this.snapshotOnly == false;
-            }
-            return featureFlag.isEnabled();
+            return enabled;
         }
 
         public String capabilityName() {
             return name().toLowerCase(Locale.ROOT);
         }
     }
 
-    public static final Set<String> CAPABILITIES = capabilities();
+    public static final Set<String> CAPABILITIES = capabilities(false);
 
-    private static Set<String> capabilities() {
+    /**
+     * Get a {@link Set} of all capabilities. If the {@code all} parameter is {@code false}
+     * then only <strong>enabled</strong> capabilities are returned - otherwise <strong>all</strong>
+     * known capabilities are returned.
+     */
+    public static Set<String> capabilities(boolean all) {
         List<String> caps = new ArrayList<>();
         for (Cap cap : Cap.values()) {
-            if (cap.isEnabled()) {
+            if (all || cap.isEnabled()) {
                 caps.add(cap.capabilityName());
             }
         }

docs/reference/esql/functions/kibana/docs/reverse.md

@@ -45,7 +45,11 @@ public class Reverse extends UnaryScalarFunction {
                 file = "string",
                 tag = "reverseEmoji",
                 description = "`REVERSE` works with unicode, too! It keeps unicode grapheme clusters together during reversal."
-            ) }
+            ) },
+        note = """
+            If Elasticsearch is running with a JDK version less than 20 then this will not properly reverse Grapheme Clusters.
+            Elastic Cloud and the JDK bundled with Elasticsearch all use newer JDKs. But if you've explicitly shifted to an older jdk
+            then you'll see things like "👍🏽😊" be reversed to  "🏽👍😊" instead of the correct "😊👍🏽"."""
     )
     public Reverse(
         Source source,

x-pack/plugin/esql/qa/testFixtures/src/main/resources/string.csv-spec

@@ -257,7 +257,7 @@ public final void test() throws Throwable {
                 assertThat(
                     "Capability is not included in the enabled list capabilities on a snapshot build. Spelling mistake?",
                     testCase.requiredCapabilities,
-                    everyItem(in(EsqlCapabilities.CAPABILITIES))
+                    everyItem(in(EsqlCapabilities.capabilities(true)))
                 );
             } else {
                 for (EsqlCapabilities.Cap c : EsqlCapabilities.Cap.values()) {