Recommend ruby/setup-ruby's bundler-cache: true option

Manually caching gems has many issues:
* Not working if there is no Gemfile.lock but only a Gemfile
* Not having the OS version in the key, which might cause binary incompatibility with system libraries of different ABI version.
* Not taking the Ruby version in account.
* Not taking the Ruby ABI version of development builds into accounts, which cannot be done with a key, but needs the commit hash.
* Using restore-keys would grow the cache over time and have extra gems in the cache.
* Those reasons are summarized in https://github.com/marketplace/actions/setup-ruby-jruby-and-truffleruby#caching-bundle-install-manually

examples.md

@@ -468,21 +468,18 @@ Replace `~/.local/share/renv` with the correct `path` if not using Ubuntu.
 
 ## Ruby - Bundler
 
-```yaml
+Caching gems with Bundler correctly is not trivial and just using `actions/cache`
-- uses: actions/cache@v2
+is [not enough](https://github.com/ruby/setup-ruby#caching-bundle-install-manually).
-  with:
+
-    path: vendor/bundle
+Instead, it is recommended to use `ruby/setup-ruby`'s
-    key: ${{ runner.os }}-gems-${{ hashFiles('**/Gemfile.lock') }}
+[`bundler-cache: true` option](https://github.com/ruby/setup-ruby#caching-bundle-install-automatically)
-    restore-keys: |
+whenever possible:
-      ${{ runner.os }}-gems-
-```
-When dependencies are installed later in the workflow, we must specify the same path for the bundler.
 
 ```yaml
-- name: Bundle install
+- uses: ruby/setup-ruby@v1
-  run: |
+  with:
-    bundle config path vendor/bundle
+    ruby-version: ...
-    bundle install --jobs 4 --retry 3
+    bundler-cache: true
 ```
 
 ## Rust - Cargo