Update runner to macos-13. (#498)

.github/workflows/unit_tests.yml

@@ -14,7 +14,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [macos-12]
+        os: [macos-13]
         podspec: [GoogleSignIn.podspec, GoogleSignInSwiftSupport.podspec]
         flag: [
           "",