Add remove_source_branch arg to create & update merge request mutation

What does this MR do and why?

Adds the remove_source_branch argument to the merge request create and update GraphQL mutation input types.

References

Screenshots or screen recordings

How to set up and validate locally

On https://gdk.test:3000/-/graphql-explorer run the following mutation:

mutation {
  mergeRequestCreate(input: {
    projectPath: "gitlab-org/gitlab-test",
    title: "Test",
    targetBranch: "master"
    sourceBranch: "spooky-stuff"
    removeSourceBranch: false
  }) {
    errors
    mergeRequest {
      forceRemoveSourceBranch
    }
  }
}

forceRemoveSourceBranch should be false. Close the merge request and then true.

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