[UII] Remove system-managed properties (dates) when updating index templates (elastic#231524)

## Summary

Resolves elastic#230902. This PR removes system-managed properties (dates) from
the retrieved index templates when updating the same index template
(triggered when experimental datastream feature settings have been
changed). The test that was failing is now re-enabled.

Similar to elastic#231505, this is due to
ES introducing these new date/timestamp fields in
elastic/elasticsearch#131536.

(cherry picked from commit 9cdbe77)

Author: jen-huang

x-pack/platform/plugins/shared/fleet/common/types/models/epm.ts

@@ -749,6 +749,13 @@ export interface IndexTemplate {
   composed_of: string[];
   ignore_missing_component_templates?: string[];
   _meta: object;
+
+  // These properties are returned on ES read operations and
+  // not allowed to be set on ES write operations
+  created_date?: number;
+  created_date_millis?: number;
+  modified_date?: number;
+  modified_date_millis?: number;
 }
 
 export interface ESAssetMetadata {

x-pack/platform/plugins/shared/fleet/server/services/package_policies/experimental_datastream_features.ts

@@ -199,8 +199,17 @@ export async function handleExperimentalDatastreamFeatureOptIn({
       });
     }
 
-    const indexTemplate = indexTemplateRes.index_templates[0].index_template;
-    let updatedIndexTemplate = indexTemplate as IndexTemplate;
+    const rawIndexTemplate = indexTemplateRes.index_templates[0].index_template;
+
+    // Remove system-managed properties (dates) that cannot be set during create/update of index templates
+    const {
+      created_date: createdDate,
+      created_date_millis: createdDateMillis,
+      modified_date: modifiedDate,
+      modified_date_millis: modifiedDateMillis,
+      ...indexTemplate
+    } = rawIndexTemplate as IndexTemplate;
+    let updatedIndexTemplate = indexTemplate;
 
     if (isTSDBOptInChanged) {
       const indexTemplateBody = {