[CI] Run buildup_SpecsTesting_repo job on macos-15 (#14506)

Author: andrewheard

.github/workflows/prerelease.yml

@@ -112,7 +112,7 @@ jobs:
     needs: [buildup_SpecsTesting_repo_FirebaseCore, specs_checking]
     # Don't run on private repo unless it is a PR.
     if: (github.repository == 'Firebase/firebase-ios-sdk' && github.event_name == 'schedule') || github.event_name == 'workflow_dispatch'
-    runs-on: macos-14
+    runs-on: macos-15
     strategy:
       fail-fast: false
       matrix: ${{fromJson(needs.specs_checking.outputs.matrix)}}