Fix GraphQL/FieldDefinitions offense (Part 2/4)

What does this MR do and why?

This fixes the GraphQL/FieldDefinitions offense in:

• app/graphql/types/merge_request_type.rb
• app/graphql/types/namespace_type.rb
• app/graphql/types/notes/note_type.rb

This MR is part 2 of a 4 part series of MRs to resolve #346029.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #346029