Fix case sensitive GraphQL username argument

What does this MR do and why?

Fix case sensitive GraphQL username argument in user query

Changelog: fixed

Screenshots or screen recordings

Before:

After:

How to set up and validate locally

Run the following GraphQL query from GrapiQL: http://gdk.test:3000/-/graphql-explorer

query getUser {
	user(username: "ROOT") {
    id
  }
}

MR acceptance checklist

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