CI: Switch to latest macOS and latest stable Xcode

Switch the CI image to the latest stable OS version, and latest stable
Xcode version, selected by another action.

Signed-off-by: Christopher Snowhill <kode54@gmail.com>

.github/workflows/debug.yml

@@ -11,10 +11,14 @@ on:
 jobs:
   build:
     name: Build Universal Cog.app
-    runs-on: macos-12
+    runs-on: macos-latest
     env:
       XCODE_DERIVEDDATA_PATH: build
     steps:
+      - name: Switch to latest stable Xcode
+        uses: maxim-lobanov/setup-xcode@v1
+        with:
+          xcode-version: latest-stable
       - name: Check out repository
         uses: actions/checkout@v2
         with: