WIP: Lock the bundler version as omnibus

What does this MR do?

Lock the bundler version as omnibus

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

This is old. Should we update the version in omnibus?

Why was this MR needed?

So that we catch omnibus-gitlab#3128 (closed) in regular tests.

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Backend

What are the relevant issue numbers?

Closes #42656 (moved)