Upgrade to Ember 6.12 stable

Move ember-source from ~6.11.0-alpha.6 to ~6.12.0 and ember-cli from
^6.10.0 to ^6.12.0. This is the final stable release of the 6.x
series and an LTS, clearing the way for a clean Ember 7 upgrade.

package.json

@@ -62,13 +62,13 @@
     "babel-plugin-ember-template-compilation": "^3.0.1",
     "concurrently": "^9.2.1",
     "decorator-transforms": "^2.4.0",
-    "ember-cli": "^6.10.0",
+    "ember-cli": "^6.12.0",
     "ember-cli-deprecation-workflow": "^4.0.1",
     "ember-modifier": "^4.3.0",
     "ember-page-title": "^9.0.3",
     "ember-qunit": "^9.1.0",
     "ember-resolver": "^13.2.0",
-    "ember-source": "~6.11.0-alpha.6",
+    "ember-source": "~6.12.0",
     "ember-template-lint": "^7.9.3",
     "ember-truth-helpers": "^5.0.0",
     "ember-welcome-page": "^8.0.4",