[ci] Remove deprecated macos-10.15 virtual environment in tests

Ref https://github.com/actions/virtual-environments/issues/5583

.github/workflows/test.yml

@@ -19,7 +19,7 @@ jobs:
       fail-fast: false
       matrix:
         go-version: ${{fromJson(needs.go-versions.outputs.versions)}}
-        os: [ubuntu-20.04, ubuntu-18.04, windows-2019, macOS-10.15, macos-11]
+        os: [ubuntu-20.04, ubuntu-18.04, windows-2019, macos-11]
     runs-on: ${{ matrix.os }}
     steps:
     - name: Install Go