Use Xcode 13.1 for macOS 11 workflow

Author: mkj-is

.github/workflows/macos-11.yml

@@ -14,6 +14,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
+    - name: Setup Xcode version
+      uses: maxim-lobanov/setup-xcode@v1.4.0
+      with:
+        xcode-version: 13.1
     - name: Lint
       run: |
         swiftlint --strict