[UA] kibana-core ownership (elastic#203410)

## Summary

As Core will be driving a lot of the development for UA for 8->9 this PR
makes `kibana-core` the temporary code owners for UA until we've
completed the 9.0 release.

.github/CODEOWNERS

@@ -2007,7 +2007,7 @@ x-pack/test/api_integration/apis/management/index_management/inference_endpoints
 # Management Experience - Deployment Management
 /test/functional/fixtures/kbn_archiver/management.json @elastic/kibana-management @elastic/kibana-data-discovery # Assigned per 2 uses: test/functional/apps/management/_import_objects.ts && test/functional/apps/management/data_views/_scripted_fields_filter.ts
 /x-pack/test/functional/fixtures/kbn_archiver/home/feature_controls/security/security.json @elastic/kibana-management
-/x-pack/test/functional/es_archives/upgrade_assistant @elastic/kibana-management
+/x-pack/test/functional/es_archives/upgrade_assistant @elastic/kibana-core
 /x-pack/test/functional/services/ace_editor.js @elastic/kibana-management
 /x-pack/test/functional/page_objects/remote_clusters_page.ts @elastic/kibana-management
 /x-pack/test/stack_functional_integration/apps/ccs @elastic/kibana-management
@@ -2033,7 +2033,7 @@ x-pack/test/api_integration/apis/management/index_management/inference_endpoints
 /x-pack/test/functional/apps/license_management @elastic/kibana-management
 /x-pack/test/functional/apps/management @elastic/kibana-management
 /x-pack/test/functional/apps/remote_clusters @elastic/kibana-management
-/x-pack/test/functional/apps/upgrade_assistant @elastic/kibana-management
+/x-pack/test/functional/apps/upgrade_assistant @elastic/kibana-core
 /x-pack/test/functional/apps/dev_tools @elastic/kibana-management
 /test/plugin_functional/test_suites/management @elastic/kibana-management
 /x-pack/test/upgrade_assistant_integration @elastic/kibana-management
@@ -2057,7 +2057,7 @@ x-pack/test/api_integration/apis/management/index_management/inference_endpoints
 /x-pack/test/api_integration/services/ingest_pipelines @elastic/kibana-management
 /x-pack/test/functional/apps/watcher @elastic/kibana-management
 /x-pack/test/api_integration/apis/watcher @elastic/kibana-management
-/x-pack/test/api_integration/apis/upgrade_assistant @elastic/kibana-management
+/x-pack/test/api_integration/apis/upgrade_assistant @elastic/kibana-core
 /x-pack/test/api_integration/apis/searchprofiler @elastic/kibana-management
 /x-pack/test/api_integration/apis/console @elastic/kibana-management
 /x-pack/test_serverless/**/test_suites/common/index_management/ @elastic/kibana-management
@@ -2090,7 +2090,7 @@ x-pack/test/api_integration/apis/management/index_management/inference_endpoints
 /x-pack/test/accessibility/apps/group3/license_management.ts @elastic/kibana-management
 /x-pack/test/accessibility/apps/group3/remote_clusters.ts @elastic/kibana-management
 /x-pack/test/accessibility/apps/group3/rollup_jobs.ts @elastic/kibana-management
-/x-pack/test/accessibility/apps/group3/upgrade_assistant.ts @elastic/kibana-management
+/x-pack/test/accessibility/apps/group3/upgrade_assistant.ts @elastic/kibana-core
 /x-pack/test/accessibility/apps/group3/watcher.ts @elastic/kibana-management
 
 #CC# /x-pack/plugins/cross_cluster_replication/ @elastic/kibana-management