Resolve "Adding a diff line note freezes the browser"

What does this MR do?

Instead of resetting the full discussion state its only updating the newly created one which fixes the problem of a full rerender.

Why was this MR needed?

Cause adding a note was resetting the whole tree which lead to a full rerender.

Does this MR meet the acceptance criteria?

• Conform by the code review guidelines
  • Has been reviewed by a Frontend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• Conform by the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #48906 (closed)