Update Codecov action to use 'files' instead of 'file'

Replaced the deprecated 'file' key with 'files' in the Codecov action
configuration. This ensures compatibility with the latest Codecov action
requirements and prevents potential upload issues.

.github/workflows/pytest-with-coverage.yaml

@@ -56,6 +56,6 @@ jobs:
       - name: Upload coverage report to Codecov
         uses: codecov/codecov-action@1e68e06f1dbfde0e4cefc87efeba9e4643565303
         with:
-          file: ./coverage.xml
+          files: ./coverage.xml
           flags: unittests
           token: ${{ secrets.CODECOV_TOKEN }}