GitHub CI: Use the latest version of macOS

Use the "macos-latest" image, otherwise we might run into support issues
with Homebrew.

[skip ci]

.github/workflows/macos.yml

@@ -5,7 +5,7 @@ on: [push]
 jobs:
   macos:
     name: Build & Test
-    runs-on: macos-11.0
+    runs-on: macos-latest
     steps:
       - name: Checkout
         uses: actions/checkout@v3