Resolve "Quick actions are case sensitive"

What does this MR do?

Makes the evaluation of quick commands case-insensitive, either by adjusting the regular expressions accordingly or by using lowercase.

Are there points in the code the reviewer needs to double check?

I'm not quite sure if the i in the regular expression in lib/gitlab/quick_actions/substitution_definition.rb is really needed. In any case, it does no harm.

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 Frontend maintainer
  • 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 #47050 (closed)