ci: Revert Bundler 4.0.6 pin and use rubygems: latest in workflows

Bundler 4.0.12 (https://blog.rubygems.org/2026/05/20/4.0.12-released.html)
includes ruby/rubygems#9544, which fixes the multi-gemspec git source
resolver regression tracked in ruby/rubygems#9536 — the bug that PR #290
worked around by pinning Bundler to 4.0.6 in the RuboCop workflow.

- Drop the BUNDLER_VERSION env var and the "Pin Bundler to 4.0.6" step
  from rubocop.yml; the workaround is no longer needed.
- Add `rubygems: latest` to the workflows that exercise modern CRuby
  (rubocop, test, test_11g, test_11g_ojdbc11), matching the pattern
  release.yml already uses, so CI resolves Bundler 4.0.12 instead of
  whatever ships in the setup-ruby tarball. setup-ruby handles JRuby
  via `gem update --system`, so JRuby matrix entries are safe.

ruby_head.yml and jruby_head.yml are intentionally left alone — per
setup-ruby docs, head builds are not updated. truffleruby.yml and
test_gemfiles.yml are also untouched (TruffleRuby compatibility, and
test_gemfiles targets Ruby 2.4–3.3 which cannot install RubyGems 4.x).

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

Author: yahonda

.github/workflows/rubocop.yml

@@ -10,21 +10,14 @@ 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

.github/workflows/test.yml

@@ -50,6 +50,7 @@ jobs:
       uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
+        rubygems: latest
     - name: Create symbolic link for libaio library compatibility
       run: |
         sudo ln -s /usr/lib/x86_64-linux-gnu/libaio.so.1t64 /usr/lib/x86_64-linux-gnu/libaio.so.1

.github/workflows/test_11g.yml

@@ -56,6 +56,7 @@ jobs:
       uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
+        rubygems: latest
     - name: Create symbolic link for libaio library compatibility
       run: |
         sudo ln -s /usr/lib/x86_64-linux-gnu/libaio.so.1t64 /usr/lib/x86_64-linux-gnu/libaio.so.1

.github/workflows/test_11g_ojdbc11.yml

@@ -52,6 +52,7 @@ jobs:
       uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
+        rubygems: latest
     - name: Create symbolic link for libaio library compatibility
       run: |
         sudo ln -s /usr/lib/x86_64-linux-gnu/libaio.so.1t64 /usr/lib/x86_64-linux-gnu/libaio.so.1