@engwan identified a race condition with how we were using
Discussion#new_discussion?

- A new discussion is created.
- A reply to the discussion is quickly posted.
- When a work is run #new_discussion? and would now return false
  because the Discussion has 2 notes.

As he suggested:

> I think we should remove Discussion#new_discussion? and instead have
> something like Note#start_of_discussion? which checks if the current
> note is the first note of a discussion.

This method replaces Discussion#new_discussion? with a more reliable
method of checking if a note is the start of a discussion, and updates
it use in the codebase.

In his review, @oswaldo identified a method with the same functionality
existed in `DiffNote#discussion_first_note?` already. This MR removes
that method, as DiffNote inherits from Note.

https://gitlab.com/gitlab-org/gitlab/issues/36329