[UA] kibana-core ownership (#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.

(cherry picked from commit 55d5c7a)

# Conflicts:
#	.github/CODEOWNERS

.github/CODEOWNERS

@@ -993,7 +993,7 @@ src/plugins/unified_histogram @elastic/kibana-data-discovery
 src/plugins/unified_search @elastic/kibana-visualizations
 packages/kbn-unsaved-changes-badge @elastic/kibana-data-discovery
 packages/kbn-unsaved-changes-prompt @elastic/kibana-management
-x-pack/plugins/upgrade_assistant @elastic/kibana-management
+x-pack/plugins/upgrade_assistant @elastic/kibana-core
 x-pack/plugins/observability_solution/uptime @elastic/obs-ux-management-team
 x-pack/plugins/drilldowns/url_drilldown @elastic/appex-sharedux
 src/plugins/url_forwarding @elastic/kibana-visualizations