Use forked github-markup gem

To use the forked version, we must undo some patching we did before.

I'm preserving one patch that changes the interpreter from python2 to python3 so that we don't change the required libraries for current source installs. This will let us release the change in a patch release instead of waiting for 8.13.

Before accepting this MR we must:

• Make sure the change in gitlab-ceis merged and it exists ingitlab-ee.
• Update the MR (gitlab-org/gitlab-ce!6659) to point to a more official / public repository, which should be in our namespace in github. (see: https://gitlab.com/gitlab-org/gitlab-ce/issues/21696#note_15231231)

Related issue: gitlab-org/gitlab-ce#21696