Don't run the downtime check on stable branches, or tags/master

What does this MR do?

Ignores the downtime check for stable branches

Why was this MR needed?

The way the check currently works, it runs on the diff between your branch and the latest commit in gitlab on master.

This doesn't give a good representation of the changes migrations for stable branches, tags, and master itself.

And is currently causing failures in the 9.0 branch

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?