Update gitlab-markup gem to 1.8.0

Changes to gem:

* Disable configuration file processing in RST
* Wrap call to rest2html in a timeout

See https://gitlab.com/gitlab-org/gitlab-markup/-/commit/9e023643c24a72089930b838aaad7bf71e70affc#d30b04166486087de34ff05cf278d3dd2ddd2360

Changelog: fixed