remove select2-rails (select2 is now installed via yarn)

What does this MR do?

Remove unused select2-rails gem.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

select2 is now installed via yarn.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Close #36711 (closed)