Upgrade actions/checkout to v4

This commit upgrades the `actions/checkout` action in the
`copy-branch.yml` workflow from v2 to v4.

Author: cartland

.github/workflows/copy-branch.yml

@@ -20,7 +20,7 @@ jobs:
     steps:
     # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it,
     # but specifies master branch (old default).
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
       with:
         fetch-depth: 0
         ref: master