Resolve "`!` autocomplete does not work anymore"

What does this MR do?

Fixes ! support to autocomplete MRs within Issue and MR comment/description fields. This was broken since %10.8 and came from https://gitlab.com/gitlab-org/gitlab-ce/commit/5c2078838bb9de710f9025513c4b6ec664bba313.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them

What are the relevant issue numbers?

Closes #48528 (closed)