[ML] Refactor deprecated routes and simplify class hierarchy (#124755)

Removes deprecated routes and API parameters.

The old 7.x routes for text file structure finding _text_structure/find_file_structure have been deprecated since 8 and replaced with _text_structure/find_structure. Also this PR simplifies the class hierarchy of GetDatafeedsStatsAction.

Non-issue as the deprecation and removal in 9 was long signaled.

Author: valeriy42

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/action/GetDatafeedsStatsAction.java

@@ -6,16 +6,15 @@
  */
 package org.elasticsearch.xpack.core.ml.action;
 
+import org.elasticsearch.action.ActionRequest;
 import org.elasticsearch.action.ActionRequestValidationException;
 import org.elasticsearch.action.ActionType;
-import org.elasticsearch.action.support.master.MasterNodeReadRequest;
 import org.elasticsearch.cluster.ClusterState;
 import org.elasticsearch.cluster.node.DiscoveryNode;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.io.stream.Writeable;
 import org.elasticsearch.core.Nullable;
-import org.elasticsearch.core.UpdateForV9;
 import org.elasticsearch.persistent.PersistentTasksCustomMetadata;
 import org.elasticsearch.tasks.CancellableTask;
 import org.elasticsearch.tasks.Task;
@@ -58,21 +57,14 @@ private GetDatafeedsStatsAction() {
         super(NAME);
     }
 
-    // This needs to be a MasterNodeReadRequest even though the corresponding transport
-    // action is a HandledTransportAction so that in mixed version clusters it can be
-    // serialized to older nodes where the transport action was a MasterNodeReadAction.
-    // TODO: Make this a simple request in a future version where there is no possibility
-    // of this request being serialized to another node.
-    @UpdateForV9(owner = UpdateForV9.Owner.MACHINE_LEARNING)
-    public static class Request extends MasterNodeReadRequest<Request> {
+    public static class Request extends ActionRequest {
 
         public static final String ALLOW_NO_MATCH = "allow_no_match";
 
         private final String datafeedId;
         private boolean allowNoMatch = true;
 
         public Request(String datafeedId) {
-            super(TRAPPY_IMPLICIT_DEFAULT_MASTER_NODE_TIMEOUT);
             this.datafeedId = ExceptionsHelper.requireNonNull(datafeedId, DatafeedConfig.ID.getPreferredName());
         }
 

x-pack/plugin/text-structure/src/main/java/org/elasticsearch/xpack/textstructure/rest/RestFindStructureAction.java

@@ -8,7 +8,6 @@
 
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.client.internal.node.NodeClient;
-import org.elasticsearch.core.UpdateForV9;
 import org.elasticsearch.rest.BaseRestHandler;
 import org.elasticsearch.rest.RestRequest;
 import org.elasticsearch.rest.Scope;
@@ -27,18 +26,9 @@
 @ServerlessScope(Scope.INTERNAL)
 public class RestFindStructureAction extends BaseRestHandler {
 
-    @UpdateForV9(owner = UpdateForV9.Owner.MACHINE_LEARNING)
-    // one or more routes use ".replaces" with RestApiVersion.V_8 which will require use of REST API compatibility headers to access
-    // that route in v9. It is unclear if this was intentional for v9, and the code has been updated to ".deprecateAndKeep" which will
-    // continue to emit deprecations warnings but will not require any special headers to access the API in v9.
-    // Please review and update the code and tests as needed. The original code remains commented out below for reference.
     @Override
     public List<Route> routes() {
-        return List.of(
-            // Route.builder(POST, BASE_PATH + "find_structure").replaces(POST, "/_ml/find_file_structure", RestApiVersion.V_8).build()
-            new Route(POST, BASE_PATH + "find_structure"),
-            Route.builder(POST, "/_ml/find_file_structure").deprecateAndKeep("Use the _text_structure API instead.").build()
-        );
+        return List.of(new Route(POST, BASE_PATH + "find_structure"));
     }
 
     @Override