Add a new MR template to update gitlab-styles

Context

We would like to provide guidelines on how to upgrade gitlab-styles in the gitlab-org/gitlab project.

What does this MR do?

Adds an MR template with a checklist to guide developers on how to achieve this.

See the rendered markdown.

Additionally, I think somebody will have to follow the steps in https://docs.gitlab.com/ee/user/project/description_templates.html#create-a-merge-request-template to activate that MR template.

Screenshots or screen recordings

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.