Icu 16546 upgrade ember for admin UI (#2793)

* v4.12.2...v5.12.0

* chore: 🤖 yarn.lock changes

* chore: 🤖 ember init blueprint changes

* chore: 🤖 yarn.lock file changes

* style: 💄 run linter and formatter

✅ Closes: https://hashicorp.atlassian.net/browse/ICU-16546

ui/admin/config/optional-features.json

@@ -2,5 +2,6 @@
   "application-template-wrapper": false,
   "default-async-observers": true,
   "jquery-integration": false,
-  "template-only-glimmer-components": true
+  "template-only-glimmer-components": true,
+  "no-implicit-route-model": true
 }