Update MacOS runner

Author: karllessard

.github/workflows/ci.yml

@@ -163,7 +163,7 @@ jobs:
           df -h
   macosx-x86_64:
     if: github.event_name == 'push' || contains(github.event.pull_request.labels.*.name, 'CI build')
-    runs-on: macos-10.15
+    runs-on: macos-11
     needs: prepare
     strategy:
       matrix: