Capitalize mentioned issue timeline notes

What does this MR do?

Renames mentioned in to Mentioned in for issue timeline notes

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

Shouldn't be any

Why was this MR needed?

Resolves existing UI inconsistency as all the other issue timeline notes have their first letter capitalized.

What are the relevant issue numbers?

Closes #21416 (closed)

Screenshots (if relevant)

Before:

After:

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • 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