bump to actions/checkout@v4

Author: dastrobu

.github/workflows/docc.yaml

@@ -9,7 +9,7 @@ jobs:
   generate-docs:
     runs-on: macos-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: generate-documentation
         # https://apple.github.io/swift-docc-plugin/documentation/swiftdoccplugin/generating-documentation-for-hosting-online/
         run: >