Add the cases when deprecations not being catched to "Ruby 3 gotchas"

What does this MR do and why?

Updates Ruby 3 gotchas doc: https://docs.gitlab.com/ee/development/ruby3_gotchas.html.
Describes the case from #364099 (closed)

This is a short-term measure to increase the awareness, I am looking into providing a more fundamental solution if such is possible.

How to set up and validate locally

Only .md is updated, rely on CI.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #364099 (closed)