deps: update actions/checkout action to v4 (#1975)

* deps: update actions/checkout action to v4

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

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Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

.github/workflows/unmanaged_dependency_check.yaml

@@ -5,7 +5,7 @@ jobs:
   unmanaged_dependency_check:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: actions/setup-java@v3
         with:
           distribution: temurin