[Infra] Run Auth quickstart on macOS 15

Author: andrewheard

.github/workflows/auth.yml

@@ -221,7 +221,7 @@ jobs:
     env:
       plist_secret: ${{ secrets.GHASecretsGPGPassphrase1 }}
       signin_secret: ${{ secrets.GHASecretsGPGPassphrase1 }}
-    runs-on: macos-14
+    runs-on: macos-15
     steps:
     - uses: actions/checkout@v4
     - uses: ruby/setup-ruby@v1