Remove github-markup in favor of gitlab-markup

What does this MR do and why?

gitlab-markup is our forked version of github-markup. There should be no reason to need the original gem, particularly at the 1.7.0 version

MR acceptance checklist

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

Relates to #379260 (closed)

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