[Deprecation API] Adjust details in the SourceFieldMapper deprecation warning (#122041)

In this PR we improve the deprecation warning about configuring source
in the mapping.

- We reduce the size of the warning message so it looks better in kibana.
- We keep the original message in the details.
- We use an alias help url, so we can associate it with the guide when it's created.

Author: gmarouli

qa/rolling-upgrade/src/javaRestTest/java/org/elasticsearch/upgrades/SourceModeRollingUpgradeIT.java

@@ -85,7 +85,13 @@ private void assertDeprecationWarningForTemplate(String templateName) throws IOE
         assertThat(response.containsKey("templates"), equalTo(true));
         Map<?, ?> issuesByTemplate = (Map<?, ?>) response.get("templates");
         assertThat(issuesByTemplate.containsKey(templateName), equalTo(true));
-        var templateIssues = (List<?>) issuesByTemplate.get(templateName);
-        assertThat(((Map<?, ?>) templateIssues.getFirst()).get("message"), equalTo(SourceFieldMapper.DEPRECATION_WARNING));
+        var templateIssue = (Map<?, ?>) ((List<?>) issuesByTemplate.get(templateName)).getFirst();
+        // Bwc compatible logic until backports are complete.
+        if (templateIssue.containsKey("details")) {
+            assertThat(templateIssue.get("message"), equalTo(SourceFieldMapper.DEPRECATION_WARNING_TITLE));
+            assertThat(templateIssue.get("details"), equalTo(SourceFieldMapper.DEPRECATION_WARNING));
+        } else {
+            assertThat(templateIssue.get("message"), equalTo(SourceFieldMapper.DEPRECATION_WARNING));
+        }
     }
 }

server/src/main/java/org/elasticsearch/index/mapper/SourceFieldMapper.java

@@ -58,6 +58,8 @@ public class SourceFieldMapper extends MetadataFieldMapper {
 
     public static final String LOSSY_PARAMETERS_ALLOWED_SETTING_NAME = "index.lossy.source-mapping-parameters";
 
+    public static final String DEPRECATION_WARNING_TITLE = "Configuring source mode in mappings is deprecated.";
+
     public static final String DEPRECATION_WARNING = "Configuring source mode in mappings is deprecated and will be removed "
         + "in future versions. Use [index.mapping.source.mode] index setting instead.";
 

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

@@ -122,9 +122,9 @@ private DeprecationIssue checkSourceModeInComponentTemplates(ComponentTemplate t
                     if (sourceMap.containsKey("mode")) {
                         return new DeprecationIssue(
                             DeprecationIssue.Level.CRITICAL,
+                            SourceFieldMapper.DEPRECATION_WARNING_TITLE,
+                            "https://ela.st/migrate-source-mode",
                             SourceFieldMapper.DEPRECATION_WARNING,
-                            "https://github.com/elastic/elasticsearch/pull/117172",
-                            null,
                             false,
                             null
                         );

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

@@ -51,9 +51,9 @@ public void testCheckSourceModeInComponentTemplates() throws IOException {
         Map<String, List<DeprecationIssue>> issuesByComponentTemplate = checker.check(clusterState);
         final DeprecationIssue expected = new DeprecationIssue(
             DeprecationIssue.Level.CRITICAL,
+            SourceFieldMapper.DEPRECATION_WARNING_TITLE,
+            "https://ela.st/migrate-source-mode",
             SourceFieldMapper.DEPRECATION_WARNING,
-            "https://github.com/elastic/elasticsearch/pull/117172",
-            null,
             false,
             null
         );