Remove Kramdown patch and update to v2.3.1 gem

As of https://gitlab.com/gitlab-org/gitlab/-/merge_requests/56750,
Markdown should no longer be rendered through Kramdown for Wiki
pages. The monkey patch can be removed with Kramdown v2.3.1.

Diff:
https://my.diffend.io/gems/kramdown/2.3.0/2.3.1