Update mac CI to use macos-11 instead of 10.15

To fix Mac CI

The macOS-10.15 environment is deprecated, consider switching to macos-11(macos-latest), macos-12 instead. For more details see https://github.com/actions/virtual-environments/issues/5583

.github/workflows/ci.yml

@@ -276,7 +276,7 @@ jobs:
   macOS:
     name: Mac
     needs: [commit-msg, clang-format]
-    runs-on: macos-10.15
+    runs-on: macos-11
     strategy:
       fail-fast: false
       matrix: