[Infra] Remove macos-13 from messaging-cron-only job

Author: andrewheard

.github/workflows/messaging.yml

@@ -230,7 +230,7 @@ jobs:
     strategy:
       matrix:
         target: [ios, tvos, macos --skip-tests, watchos --skip-tests]
-        os: [macos-14, macos-13]
+        os: [macos-14, macos-15]
         include:
           - os: macos-15
             xcode: Xcode_16.2