Fix merge request Draft title not appearing

What does this MR do and why?

!88964 (merged) accidentally dropped the title class from the merge request title header, preventing the JavaScript in issuable_form.js from locating the title to add the Draft prefix.

See the subtle change:

Closes #364720 (closed)

How to set up and validate locally

Run bundle exec rspec ./spec/features/merge_request/user_marks_merge_request_as_draft_spec.rb

MR acceptance checklist

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