Solve Bundler version mismatch between rails and CNG

Summary

As seen in Update rubygems/rubygems from 3.2.33 to 3.4.3 (gitlab-org/build/CNG!1259 - merged), there can be a mismatch in the Bundler version between rails and Cloud Native GitLab if the upstream bot makes a silent change.

Initial investigation was documented in the original merge request.

Expected outcome

Identify the root cause and implement a stable, maintainable solution to keep the Bundler versions aligned.