fix: Use correct parameter name 'files' for codecov-action@v5

Change 'file' to 'files' (plural) as required by codecov-action@v5
to fix the 'Unexpected input' warning.

Author: github-actions

.github/workflows/tests.yml

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