CI: macos-10.15 -> macos-11 (#9087)

macOS-10.15 deprecated and remove soon GitHub Actions.
We needs migrate to macos-11 (Big Sur)

Author: zeriyoshi

.github/workflows/nightly.yml

@@ -105,7 +105,7 @@ jobs:
         debug: [true, false]
         zts: [true, false]
     name: "${{ matrix.branch.name }}_MACOS_${{ matrix.debug && 'DEBUG' || 'RELEASE' }}_${{ matrix.zts && 'ZTS' || 'NTS' }}"
-    runs-on: macos-10.15
+    runs-on: macos-11
     steps:
       - name: git checkout
         uses: actions/checkout@v2

.github/workflows/push.yml

@@ -60,7 +60,7 @@ jobs:
       - name: Verify generated files are up to date
         uses: ./.github/actions/verify-generated-files
   MACOS_DEBUG_NTS:
-    runs-on: macos-10.15
+    runs-on: macos-11
     steps:
       - name: git checkout
         uses: actions/checkout@v2