Migrate form checkbox to gitlab UI style in MR settings

Mike Greilingrequested to merge
mg-update-merge-trains-settings-checkbox into master

What does this MR do and why?

Migrates the vanilla form helper checkbox element to the GitLab UI styled checkbox using the gitlab_ui_checkbox_component helper.

Screenshots or screen recordings

before after
CleanShot_2022-04-08_at_01.54.14_2x CleanShot_2022-04-08_at_01.57.49_2x

How to set up and validate locally

Visit a project settings -> general page, expand the "merge requests" section and observe the checkbox style and behavior.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Merge request reports