Add stable branch template

What does this MR do?

Add a merge request template to be used when backporting bug fixes into Omnibus stable branches.

Related issues

Related to gitlab-com/gl-infra/delivery#2886 (closed) and to gitlab-com/gl-infra/delivery#2890 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• [-] Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• [-] trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• [-] Test plan indicating conditions for success has been posted and passes
• [-] Documentation created/updated
• [-] Tests added
• [-] Integration tests added to GitLab QA
• [-] Equivalent MR/issue for the GitLab Chart opened