Use macos-11 in CI workflow

See: https://github.com/actions/virtual-environments/issues/2638

.github/workflows/ci.yaml

@@ -8,7 +8,7 @@ jobs:
 
     strategy:
       matrix:
-        os: [ ubuntu-latest, macos-latest, windows-latest ]
+        os: [ ubuntu-latest, macos-11, windows-latest ]
 
     steps:
       - uses: actions/checkout@v2
@@ -38,7 +38,7 @@ jobs:
         if: matrix.os == 'windows-latest'
 
       - run: ./gradlew build
-        if: matrix.os == 'macos-latest'
+        if: matrix.os == 'macos-11'
 
   publish:
     needs: build
@@ -47,7 +47,7 @@ jobs:
 
     strategy:
       matrix:
-        os: [ ubuntu-latest, macos-latest, windows-latest ]
+        os: [ ubuntu-latest, macos-11, windows-latest ]
 
     env:
       ORG_GRADLE_PROJECT_ossrhUsername: ${{ secrets.OSSRH_USERNAME }}
@@ -84,4 +84,4 @@ jobs:
         if: matrix.os == 'windows-latest'
 
       - run: ./gradlew publish
-        if: matrix.os == 'macos-latest'
+        if: matrix.os == 'macos-11'