Fix up the alternative command for deprecated gdk thin

What does this Merge Request do and why?

Ask users to use GITLAB_RAILS_RACK_TIMEOUT_ENABLE_LOGGING=false gdk rails s for the alternative for gdk thin.

cf. https://guides.rubyonrails.org/command_line.html

Fixes up !2591 (merged)

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• [n/a] Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• [n/a] gdk doctor test added, if needed.
• [n/a] Add the ~highlight label if this MR should be included in the CHANGELOG.md.