Migrate sourcegraph checkboxes to gitlab-ui

What does this MR do and why?

Migrate sourcegraph checkboxes to gitlab-ui

Screenshots or screen recordings

before	after

How to set up and validate locally

Admin > Settings > General > Sourcegraph

MR acceptance checklist

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

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