GitHub Actions: Remove macos-12 job, as the image will disappear soon (#9085)

Author: kinke

.github/workflows/main.yml

@@ -24,8 +24,6 @@ jobs:
           # macOS
           - job_name: macOS 13 x64
             os: macos-13
-          - job_name: macOS 12 x64
-            os: macos-12
           # Windows
           - job_name: Windows x64
             os: windows-2022
@@ -83,13 +81,6 @@ jobs:
         with:
           arch: ${{ env.MODEL == '64' && 'x64' || 'x86' }}
 
-      # NOTE: Linker ICEs with Xcode 15.0.1 (default version on macos-13)
-      # * https://issues.dlang.org/show_bug.cgi?id=24407
-      # Remove this step if the default gets changed to 15.1 in actions/runner-images.
-      - name: 'macOS 13: Switch to Xcode v15.1'
-        if: matrix.os == 'macos-13'
-        run: sudo xcode-select -switch /Applications/Xcode_15.1.app
-
       - name: 'Posix: Install host compiler'
         if: runner.os != 'Windows'
         run: cd ../dmd && ci/run.sh install_host_compiler