Add deprecation warning for openSUSE Leap 15.1

What does this MR do?

Announce that openSUSE 15.1 is deprecated and will be removed starting with 13.7.

Related issues

#5688 (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

Closes #5688 (closed)