github workflow: update macos version to macos-11

Author: martinhsv

.github/workflows/ci.yml

@@ -46,7 +46,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [macos-10.15]
+        os: [macos-11]
         compiler: [clang]
         configure:
           - {label: "with parser generation", opt: "--enable-parser-generation" }