Bump codecov/codecov-action from 3 to 4 (#28)

* 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] <support@github.com>

* Use Codecov upload token.

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Andrew Walbran <qwandor@google.com>

.github/workflows/rust.yml

@@ -43,7 +43,8 @@ jobs:
       - name: Run tests with coverage
         run: cargo llvm-cov test --all-features --codecov --output-path codecov-report.json
       - name: Upload coverage to codecov.io
-        uses: codecov/codecov-action@v3
+        uses: codecov/codecov-action@v4
         with:
-          files: codecov-report.json
           fail_ci_if_error: true
+          files: codecov-report.json
+          token: ${{ secrets.CODECOV_TOKEN }}