ci: Drop Bundler 4.0.6 pin now that Bundler 4.0.12 ships the fix

The RuboCop workflow pinned Bundler to 4.0.6 in #2767 to dodge
ruby/rubygems#9536, which broke BUNDLE_ONLY-filtered resolves against
the multi-gemspec rails/rails git source. ruby/rubygems#9544 fixed the
regression, and Bundler 4.0.12 (2026-05-20) ships the fix:
https://blog.rubygems.org/2026/05/20/4.0.12-released.html

Remove the workaround and let setup-ruby's `rubygems: latest` pick up
the fixed Bundler. Also add `rubygems: latest` to linting.yml for
consistency with every other workflow that runs setup-ruby.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: yahonda

.github/workflows/linting.yml

@@ -11,6 +11,7 @@ jobs:
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: 4.0
+          rubygems: latest
       - name: Yamllint
         uses: karancode/yamllint-github-action@master
         with:

.github/workflows/rubocop.yml

@@ -10,20 +10,13 @@ jobs:
   build:
 
     runs-on: ubuntu-latest
-    env:
-      BUNDLER_VERSION: 4.0.6
     steps:
       - uses: actions/checkout@v6
       - name: Set up Ruby 4.0
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: 4.0
-      - name: Pin Bundler to 4.0.6 (workaround for ruby/rubygems#9536)
-        run: |
-          gem install bundler -v 4.0.6
-          ruby --version
-          gem --version
-          bundle --version
+          rubygems: latest
       - name: Build and run RuboCop
         run: |
           BUNDLE_ONLY=rubocop bundle install --jobs 4 --retry 3