Use macos-latest again

.github/workflows/ci.yaml

@@ -48,7 +48,7 @@ jobs:
     runs-on:  ${{ matrix.os }}
     strategy:
       matrix:
-        os: [macos-11, ubuntu-latest, windows-latest]
+        os: [macos-latest, ubuntu-latest, windows-latest]
       fail-fast: false
     steps:
       - uses: actions/checkout@v2