Change Danger failure to warning for backports

What does this MR do and why?

This cherry picks two commits:

• 75ad1095
• 5facc548

!104870 (merged) was added in 15.7. It causes Danger to fail on any MRs targeting versions outside the extended maintenance policy (last three versions). In 15.9, we decided to change that failure to just a warning because we need the ability to merge into older branches. That was done with these two commits. This means that MRs targeting 15-8-stable-ee and 15-7-stable-ee will still fail due to the original rule. So here we backport the update into 15-7-stable-ee.

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