Bump codecov/codecov-action from 3 to 4 (#955)

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]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/ci.yaml

@@ -54,6 +54,6 @@ jobs:
         env:
           DFTK_TEST_ARGS: ${{ matrix.payload }}
       - uses: julia-actions/julia-processcoverage@v1
-      - uses: codecov/codecov-action@v3
+      - uses: codecov/codecov-action@v4
         with:
           files: lcov.info