Clarify flaky failures should be picked into stable branches

Why is this change being made?

Delivery is working to extend the GitLab maintenance policy to support bug fixes to the two previous monthly releases in addition to the current stable release (details on gitlab-com/gl-infra&828 (closed)). Part of this work will open up the stable branches to allow developers to merge bug fixes directly into the stable branches.

Stable branches represent the source of GitLab releases, so it is vital to guarantee their readiness and availability. This MR reinforces the idea of cherry-picking flaky failures into stable branches as a way to prevent future failures on these branches.

Related to gitlab-com/gl-infra/delivery#2637 (closed)

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