Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/yank-gem-version.yml

@@ -14,7 +14,7 @@ jobs:
     env:
       BUNDLE_WITHOUT: optional:development:special
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Setup Ruby
         uses: ruby/setup-ruby@v1