update the macOS CI image to macOS-11

Author: DimitrisStaratzis

.github/workflows/github_actions.yml

@@ -40,7 +40,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ ubuntu-latest, macos-10.15 ]
+        os: [ ubuntu-latest, macos-11 ]
 
     steps:
       - name: checkout
@@ -168,7 +168,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ macos-10.15, ubuntu-latest ]
+        os: [ macos-11, ubuntu-latest ]
     steps:
       - name: checkout
         uses: actions/checkout@v2