[github] Don't use the now-retired macos-13 runner.

Author: neboat

.github/workflows/small-cilkapps.yml

@@ -25,7 +25,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macOS-13, macOS-latest]
+        os: [ubuntu-latest, macOS-latest]
     steps:
     - name: checkout
       uses: actions/checkout@v4