Fix diff comments not showing up in activity feed

What does this MR do?

It fixes the detection of note events to check for Note and LegacyDiffNote.

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

No? /cc @DouweM (since I believe you introduced LegacyDiffNote

Why was this MR needed?

To fix #19092 (closed).

What are the relevant issue numbers?

Fixes #19092 (closed).

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together