[rubygems/rubygems] Bump actions/checkout to v4 in bundler gem template

https://github.com/rubygems/rubygems/commit/5ed4c600da

lib/bundler/templates/newgem/github/workflows/main.yml.tt

@@ -17,7 +17,7 @@ jobs:
           - '<%= RUBY_VERSION %>'
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
 <%- if config[:ext] == 'rust' -%>
     - name: Set up Ruby & Rust
       uses: oxidize-rb/actions/setup-ruby-and-rust@v1