ci: Upgrade actions/checkout from v1 to v6

The v1 action runs on an end-of-life Node runtime. v6 brings the current
runtime plus improved submodule and credential handling. The recursive
submodule checkout input is unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: macmade

.github/workflows/ci-mac.yaml

@@ -10,7 +10,7 @@ jobs:
                     - { scheme: 'Xcode Format', configuration: 'Release', project: 'XcodeFormat.xcodeproj', build: 1, analyze: 1, test: 0, info: 1, destination: 'platform=macOS' }
         steps:
 
-            - uses: actions/checkout@v1
+            - uses: actions/checkout@v6
               with:
                 submodules: 'recursive'