Create todos only for new mentiones

What does this MR do?

Doesn't create new todos when a description (of issues or MRs or note content) has changed for users who were already mentioned before. It creates new todos only for newly mentioned users.

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

The TodoService probably needs some refactoring but I'd create a separate issue for that.

Why was this MR needed?

There was a bug that new todos were created for all users mentioned in a description in case the description has changed - also for users who were mentioned in that description already before that change.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #28799 (closed)