Fix text for the merge request lock system note

What does this MR do?

It creates a correct system note in case a merge request was locked.

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

Not sure we should create a migration. RC2 where this fix should be included in the ideal case might follow very soon after RC2. There might be no usage of this new feature.

Why was this MR needed?

We added MR lock system notes later and forgot to update the text.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #38941 (closed)