revert: Change back to 'file' parameter for coverage upload

Revert 'files' to 'file' as coverage upload stopped working.
The 'file' parameter may still work despite the deprecation warning.

Author: github-actions

.github/workflows/tests.yml

@@ -68,7 +68,7 @@ jobs:
         uses: codecov/codecov-action@v5
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
-          files: ./coverage.xml
+          file: ./coverage.xml
           flags: unittests
           name: codecov-umbrella
           fail_ci_if_error: false