Bump codecov/codecov-action from 3 to 4 (#127)

* 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]>

* Add codecov secret

---------

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Hofer-Julian <[email protected]>

.github/workflows/CI.yml

@@ -34,7 +34,7 @@ jobs:
         # This is necessary until proper error reporting is implemented by Modflow
         run: |
           pytest -vs --cov=xmipy --cov-report=xml tests/
-        env:
-          GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"
       - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@v3
+        uses: codecov/codecov-action@v4
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}