Update actions/cache action to v6

Author: renovate[bot]

.github/workflows/test.yml

@@ -37,7 +37,7 @@ jobs:
         with:
           python-version: '3.13'
       - name: Cache pip
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           # This path is specific to Ubuntu
           path: ~/.cache/pip