Introduce a stable branch MR template

What does this MR do and why?

Introduce a stable branch MR template

As part of extending the maintenance policy, developers will be enabled to merge into stable branches so they can self-serve on bug fixes. The Default merge request template is not aligned with the purpose of merging bug fixes into stable branches: These fixes should have been merged into the default branch and deployed to GitLab.com.

This commit introduces a merge request template for stable branches defining the guidelines to follow when merging into these branches.

Related to gitlab-com/gl-infra/delivery#2839 (closed)

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.