Bump actions/cache to v4

    The following actions use a deprecated Node.js version and will be forced to
    run on node20: actions/cache@v3. For more info:
    https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

.github/workflows/ci.yml

@@ -82,7 +82,7 @@ jobs:
         run: bundle lock
 
       - name: Cache dependencies
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           path: vendor/bundle
           key: bundle-${{ hashFiles('Gemfile.lock') }}