Rename file key to codecov-action to files

Author: jwodder

.github/workflows/test.yml

@@ -139,7 +139,7 @@ jobs:
       uses: codecov/codecov-action@v5
       with:
         token: ${{ secrets.CODECOV_TOKEN }}
-        file: ./coverage.xml
+        files: ./coverage.xml
         flags: unittests
         # name: codecov-umbrella
         # yml: ./codecov.yml