Downgrade macOS/Xcode to test code backwards compatibility

Author: Paulchen-Panther

.github/workflows/qt5_6.yml

@@ -97,7 +97,7 @@ jobs:
 
   macOS:
     name: 🍏 macOS x64
-    runs-on: macos-13
+    runs-on: macos-12
     steps:
       - name: ⬇ Checkout
         uses: actions/checkout@v4
@@ -128,6 +128,13 @@ jobs:
             python3 --version
           echo '::endgroup::'
 
+          echo '::group::Set up Xcode'
+            # Oldest version of Xcode supported on GitHub Action to test source code backwards compatibility
+            sudo xcode-select -s /Applications/Xcode_13.2.1.app/Contents/Developer
+            xcode-select -p
+            xcodebuild -version
+          echo '::endgroup::'
+
       - name: 👷 Build
         shell: bash
         run: ./.github/scripts/build.sh