Script for migrating .watches and .triggered_watches indices (elastic#120371)

Introduces a script for reindexing .watches and .triggered_watches indices from V7, removing unused `types` field from documents if it exists.

Author: alexey-ivanov-es

docs/changelog/120371.yaml

@@ -0,0 +1,5 @@
+pr: 120371
+summary: Script for migrating `.watches` and `.triggered_watches` indices
+area: "Watcher"
+type: upgrade
+issues: []

x-pack/plugin/watcher/src/main/java/org/elasticsearch/xpack/watcher/Watcher.java

@@ -285,6 +285,13 @@ public class Watcher extends Plugin implements SystemIndexPlugin, ScriptPlugin,
      * to these old nodes that the mappings are newer than they are.
      */
     private static final String LEGACY_VERSION_FIELD_VALUE = "8.12.0";
+
+    private static final String REINDEX_SCRIPT_FROM_V7 = """
+        if (ctx._source.input?.search?.request?.types != null) {
+          ctx._source.input.search.request.remove('types');
+        }
+        """;
+
     private WatcherIndexingListener listener;
     private HttpClient httpClient;
     private BulkProcessor2 bulkProcessor;
@@ -808,6 +815,7 @@ public Collection<SystemIndexDescriptor> getSystemIndexDescriptors(Settings sett
                 .setVersionMetaKey("version")
                 .setOrigin(WATCHER_ORIGIN)
                 .setIndexFormat(6)
+                .setMigrationScript(REINDEX_SCRIPT_FROM_V7)
                 .build(),
             SystemIndexDescriptor.builder()
                 .setIndexPattern(TriggeredWatchStoreField.INDEX_NAME + "*")
@@ -818,6 +826,7 @@ public Collection<SystemIndexDescriptor> getSystemIndexDescriptors(Settings sett
                 .setVersionMetaKey("version")
                 .setOrigin(WATCHER_ORIGIN)
                 .setIndexFormat(6)
+                .setMigrationScript(REINDEX_SCRIPT_FROM_V7)
                 .build()
         );
     }