Update actions/cache action to v6

Author: renovate[bot]

.github/workflows/pages.yaml

@@ -47,7 +47,7 @@ jobs:
       # It is also used to cache the generated social media cards.
       # Persisting the cache across builds dramatically speeds up the process.
       - name: Load site cache
-        uses: actions/cache@0057852bfaa89a56745cba8c7296529d2fc39830 # v4
+        uses: actions/cache@2c8a9bd7457de244a408f35966fab2fb45fda9c8 # v6
         with:
           key: mkdocs
           path: .cache