Upgrade cache action from v2 to v4

Author: TryKatChup

.github/workflows/github-pages.yml

@@ -12,7 +12,7 @@ jobs:
       - uses: actions/checkout@v2
 
       # Use GitHub Actions' cache to cache dependencies on servers
-      - uses: actions/cache@v2
+      - uses: actions/cache@v4
         with:
           path: vendor/bundle
           key: ${{ runner.os }}-gems-${{ hashFiles('**/Gemfile.lock') }}