Up the frozen indices deprecation to CRITICAL (elastic#119879)

We will be removing the ability to read frozen indices in 9.0, so we
should issue a critical level deprecation issue in 8.18 and 8.19.

ES-9736 #comment Upped the deprecation level to CRITICAL

Author: PeteGillinElastic

docs/changelog/119879.yaml

@@ -0,0 +1,14 @@
+pr: 119879
+summary: Increase the frozen indices deprecation level to `CRITICAL`
+area: Indices APIs
+type: deprecation
+issues: []
+deprecation:
+  title: Increase the frozen indices deprecation level to `CRITICAL`
+  area: Index setting
+  details: >-
+    The migration deprecations API previously returned a warning on frozen indices. Support for reading frozen indices
+    will be removed in 9.0, so in 8.18 this has been made a critical issue.
+  impact: >-
+    Users are required to unfreeze any frozen indices before upgrading to 9.x. (N.B. It was impossible to freeze indices
+    in 8.x, so this only applies to 7.x indices which have not been reindexed.)

x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/IndexDeprecationChecks.java

@@ -119,12 +119,12 @@ static DeprecationIssue frozenIndexSettingCheck(IndexMetadata indexMetadata, Clu
         if (Boolean.TRUE.equals(isIndexFrozen)) {
             String indexName = indexMetadata.getIndex().getName();
             return new DeprecationIssue(
-                DeprecationIssue.Level.WARNING,
-                "index ["
-                    + indexName
-                    + "] is a frozen index. The frozen indices feature is deprecated and will be removed in a future version",
+                DeprecationIssue.Level.CRITICAL,
+                "Index [" + indexName + "] is a frozen index. The frozen indices feature is deprecated and will be removed in version 9.0.",
                 "https://www.elastic.co/guide/en/elasticsearch/reference/master/frozen-indices.html",
-                "Frozen indices no longer offer any advantages. Consider cold or frozen tiers in place of frozen indices.",
+                "Frozen indices must be unfrozen before upgrading to version 9.0."
+                    + " (The legacy frozen indices feature no longer offers any advantages."
+                    + " You may consider cold or frozen tiers in place of frozen indices.)",
                 false,
                 null
             );

x-pack/plugin/deprecation/src/test/java/org/elasticsearch/xpack/deprecation/IndexDeprecationChecksTests.java

@@ -223,10 +223,12 @@ public void testFrozenIndex() {
             issues,
             contains(
                 new DeprecationIssue(
-                    DeprecationIssue.Level.WARNING,
-                    "index [test] is a frozen index. The frozen indices feature is deprecated and will be removed in a future version",
+                    DeprecationIssue.Level.CRITICAL,
+                    "Index [test] is a frozen index. The frozen indices feature is deprecated and will be removed in version 9.0.",
                     "https://www.elastic.co/guide/en/elasticsearch/reference/master/frozen-indices.html",
-                    "Frozen indices no longer offer any advantages. Consider cold or frozen tiers in place of frozen indices.",
+                    "Frozen indices must be unfrozen before upgrading to version 9.0."
+                        + " (The legacy frozen indices feature no longer offers any advantages."
+                        + " You may consider cold or frozen tiers in place of frozen indices.)",
                     false,
                     null
                 )