chore: migrate codeowners to @grafana/grafana-catalog (#220)

* chore: migrate codeowners to @grafana/grafana-catalog

* chore: split frontend-platform-owned actions to @grafana/grafana-frontend-platform

Based on contribution history (Jack Westbrook, Hugo Häggmark dominate these
directories), route bundle-schema-types, bundle-size, bundle-types, and
create-plugin-update to the frontend-platform team. Everything else stays
under @grafana/grafana-catalog as the default.

Author: tolzhabayev

.github/CODEOWNERS

@@ -2,4 +2,10 @@
 # the repo. Unless a later match takes precedence,
 # these will be requested for review when someone opens a pull request.
 
-* @grafana/plugins-platform-frontend @grafana/plugins-platform-backend
+* @grafana/grafana-catalog
+
+# Frontend-platform-owned actions (based on contribution history)
+/bundle-schema-types/  @grafana/grafana-frontend-platform
+/bundle-size/          @grafana/grafana-frontend-platform
+/bundle-types/         @grafana/grafana-frontend-platform
+/create-plugin-update/ @grafana/grafana-frontend-platform