GH Actions: version update for actions/cache

The update doesn't actually contain any changed functionality, it's mostly just a change of the Node version used by the action itself (from Node 12 to Node 16), but see the changelog for more info.

Refs:
* https://github.com/actions/cache/releases/

.github/workflows/php.yml

@@ -42,7 +42,7 @@ jobs:
           key: ${{ env.key }}
 
       - name: Cache extensions
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: ${{ steps.cache-env.outputs.dir }}
           key: ${{ steps.cache-env.outputs.key }}