Disabling the migration reindex yaml rest tests unless feature is available (elastic#118382) (elastic#118392)

This disables the migration reindex yaml rest tests when the feature
flag is not available, so that the build doesn't fail in release mode,
like:

```
./gradlew ":x-pack:plugin:yamlRestTest" --tests "org.elasticsearch.xpack.test.rest.XPackRestIT.test {p0=migrate/*}" -Dtests.seed=28C1E85204B9DCAE -Dbuild.snapshot=false -Dtests.jvm.argline="-Dbuild.snapshot=false" -Dlicense.key=x-pack/license-tools/src/test/resources/public.key -Dtests.locale=bn-BD -Dtests.timezone=America/Dawson -Druntime.java=23
```

Author: masseyke

x-pack/plugin/migrate/src/main/java/org/elasticsearch/xpack/migrate/rest/RestMigrationReindexAction.java

@@ -20,11 +20,16 @@
 import org.elasticsearch.xpack.migrate.action.ReindexDataStreamAction.ReindexDataStreamResponse;
 
 import java.io.IOException;
+import java.util.Collections;
+import java.util.HashSet;
 import java.util.List;
+import java.util.Set;
 
 import static org.elasticsearch.rest.RestRequest.Method.POST;
+import static org.elasticsearch.xpack.migrate.action.ReindexDataStreamAction.REINDEX_DATA_STREAM_FEATURE_FLAG;
 
 public class RestMigrationReindexAction extends BaseRestHandler {
+    public static final String MIGRATION_REINDEX_CAPABILITY = "migration_reindex";
 
     @Override
     public String getName() {
@@ -49,6 +54,15 @@ protected RestChannelConsumer prepareRequest(RestRequest request, NodeClient cli
         );
     }
 
+    @Override
+    public Set<String> supportedCapabilities() {
+        Set<String> capabilities = new HashSet<>();
+        if (REINDEX_DATA_STREAM_FEATURE_FLAG.isEnabled()) {
+            capabilities.add(MIGRATION_REINDEX_CAPABILITY);
+        }
+        return Collections.unmodifiableSet(capabilities);
+    }
+
     static class ReindexDataStreamRestToXContentListener extends RestBuilderListener<ReindexDataStreamResponse> {
 
         ReindexDataStreamRestToXContentListener(RestChannel channel) {

x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/migrate/10_reindex.yml

@@ -6,6 +6,13 @@ setup:
 
 ---
 "Test Reindex With Unsupported Mode":
+  - requires:
+      reason: "migration reindex is behind a feature flag"
+      test_runner_features: [capabilities]
+      capabilities:
+        - method: POST
+          path: /_migration/reindex
+          capabilities: [migration_reindex]
   - do:
       catch: /illegal_argument_exception/
       migrate.reindex:
@@ -19,6 +26,13 @@ setup:
 
 ---
 "Test Reindex With Nonexistent Data Stream":
+  - requires:
+      reason: "migration reindex is behind a feature flag"
+      test_runner_features: [capabilities]
+      capabilities:
+        - method: POST
+          path: /_migration/reindex
+          capabilities: [migration_reindex]
   - do:
       catch: /resource_not_found_exception/
       migrate.reindex:
@@ -44,6 +58,13 @@ setup:
 
 ---
 "Test Reindex With Bad Data Stream Name":
+  - requires:
+      reason: "migration reindex is behind a feature flag"
+      test_runner_features: [capabilities]
+      capabilities:
+        - method: POST
+          path: /_migration/reindex
+          capabilities: [migration_reindex]
   - do:
       catch: /illegal_argument_exception/
       migrate.reindex:
@@ -57,6 +78,13 @@ setup:
 
 ---
 "Test Reindex With Existing Data Stream":
+  - requires:
+      reason: "migration reindex is behind a feature flag"
+      test_runner_features: [capabilities]
+      capabilities:
+        - method: POST
+          path: /_migration/reindex
+          capabilities: [migration_reindex]
   - do:
       indices.put_index_template:
         name: my-template1

x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/migrate/20_reindex_status.yml

@@ -6,13 +6,27 @@ setup:
 
 ---
 "Test get reindex status with nonexistent task id":
+  - requires:
+      reason: "migration reindex is behind a feature flag"
+      test_runner_features: [capabilities]
+      capabilities:
+        - method: POST
+          path: /_migration/reindex
+          capabilities: [migration_reindex]
   - do:
       catch: /resource_not_found_exception/
       migrate.get_reindex_status:
         index: "does_not_exist"
 
 ---
 "Test Reindex With Existing Data Stream":
+  - requires:
+      reason: "migration reindex is behind a feature flag"
+      test_runner_features: [capabilities]
+      capabilities:
+        - method: POST
+          path: /_migration/reindex
+          capabilities: [migration_reindex]
   - do:
       indices.put_index_template:
         name: my-template1