Upgrade GitHub Actions to macOS 11 (erigontech#3054)

Author: AlexeyAkhunov

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
   tests:
     strategy:
       matrix:
-        os: [ ubuntu-20.04, macos-10.15 ] # list of os: https://github.com/actions/virtual-environments
+        os: [ ubuntu-20.04, macos-11 ] # list of os: https://github.com/actions/virtual-environments
     runs-on: ${{ matrix.os }}
 
     steps: