Release CI: macOS job on macos-15-xlarge

Custom macOS larger runners in a runner group do not pick up jobs; the
built-in M2 Pro label does. Only release.yml uses it, and the
actions_macos_xl budget caps the monthly spend.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/release.yml

@@ -74,7 +74,7 @@ jobs:
   macos:
     name: macOS universal
     needs: version
-    runs-on: macos-release  # org larger runner (M2 Pro), release.yml@release only; capped by the actions_macos_xl budget
+    runs-on: macos-15-xlarge  # M2 Pro larger runner; use only in release.yml (actions_macos_xl budget caps spend)
     environment: release
     env:
       DECKCRAFT_VERSION: ${{ needs.version.outputs.version }}