Bump codecov/codecov-action from 5 to 6

Bumps [codecov/codecov-action](https://github.com/codecov/codecov-action) from 5 to 6.
- [Release notes](https://github.com/codecov/codecov-action/releases)
- [Changelog](https://github.com/codecov/codecov-action/blob/main/CHANGELOG.md)
- [Commits](codecov/codecov-action@v5...v6)

---
updated-dependencies:
- dependency-name: codecov/codecov-action
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -339,7 +339,7 @@ jobs:
                --output-file lcov.info
 
       - name: Upload coverage data (Codecov)
-        uses: codecov/codecov-action@v5
+        uses: codecov/codecov-action@v6
         if: ${{ matrix.test_type == 'coverage' }}
         with:
           files: ./libtrixi-julia/lcov.info,./lcov.info