fix: update Codecov action configuration

- Change 'file' parameter to 'files' for codecov-action@v5
- The v5 action requires 'files' (plural) instead of 'file'
- This fixes CI failures across all Python versions

Author: jdrhyne

.github/workflows/ci.yml

@@ -50,7 +50,7 @@ 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