Bump codecov/codecov-action from 3 to 4

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

---
updated-dependencies:
- dependency-name: codecov/codecov-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

.github/workflows/test.yaml

@@ -75,4 +75,4 @@ jobs:
           sudo -E "PATH=$PATH" bash -c "pytest --cov=systemdspawner --system-test-user=$(whoami)"
 
       # GitHub action reference: https://github.com/codecov/codecov-action
-      - uses: codecov/codecov-action@v3
+      - uses: codecov/codecov-action@v4