Bump actions/checkout from 3 to 4

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...v4)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/ci.yaml

@@ -173,13 +173,13 @@ jobs:
         run: git -c "http.https://github.com/.extraheader=" fetch --recurse-submodules=no "https://github.com/lampepfl/dotty" && git reset --hard FETCH_HEAD || true
 
       - name: Checkout cleanup script
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Cleanup
         run: .github/workflows/cleanup.sh
 
       - name: Git Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Add SBT proxy repositories
         run: cp -vf .github/workflows/repositories /root/.sbt/ ; true