ci: pin GitHub Actions macOS runners to macos-15

Co-authored-by: Michi Hoffmann <michael.hoffmann@sentry.io>

Author: sentry-junior[bot]

.github/workflows/workflow-tests.yml

@@ -111,7 +111,7 @@ jobs:
 
   # Test no-change scenario - should detect no updates needed
   updater-no-changes:
-    runs-on: macos-latest
+    runs-on: macos-15
     steps:
       - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4