bump actions/cache to v4

Author: zkat

.github/workflows/gh-pages.yml

@@ -20,7 +20,7 @@ jobs:
           node-version: '20.x'
 
       - name: Cache dependencies
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           path: ~/.npm
           key: ${{ runner.os }}-node-${{ hashFiles('**/package-lock.json') }}