[2024-03] Address comments on "selection set" PR so that it can be merged

[benjie]

Mutation is missing variables.

• assignee(s): @benjie
• source: https://github.com/graphql/graphql-wg/blob/2b81dcd0eb600e5cb96427ce5864ea5d07965b63/notes/2024/2024-03.md#get-some-of-benjies-editorial-prs-merged-15m-benjie

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