updated cache version

.github/workflows/Test-CI-main.yml

@@ -90,7 +90,7 @@ jobs:
 
       # The caching we do follows from
       #   https://stackoverflow.com/questions/59127258/how-can-i-use-pip-cache-in-github-actions
-      - uses: actions/cache@v2
+      - uses: actions/cache@v4
         id: cache-pip
         with:
           path: ~/.cache/pip