Merge branch 'security-untrusted-math-15-11' into '15-11-stable-ee'

Use UntrustedRegexp to protect MathFilter regex

See merge request https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/3251



Merged-by: GitLab Release Tools Bot <delivery-team+release-tools@gitlab.com>
Approved-by: Kerri Miller <kerrizor@kerrizor.com>
Co-authored-by: Brett Walker <bwalker@gitlab.com>