Mermaid Gantt Chart does not render in Milestone

Summary

Mermaid Gantt Chart does not render in Milestone since the last 2 Gitlab releases It works fine in a normal issue or even epics and has worked fine before.

Steps to reproduce

1. Create a mermaid gantt chart in a milestone issue
2. In Preview the chart will be rendered correctly
3. Once you save the MD you will see a box containing the Mermaid text instead of a rendered chart

Example Project

What is the current bug behavior?

Seeing source code instead of rendered mermaid char

What is the expected correct behavior?

Mermaid gantt chart is rendered

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes