Skip package-and-test message on trivial MRs

What does this MR do and why?

The execution of package-and-test pipeline is required on merge request targeting stable branches as a way to guarantee the changes introduced are compliant from the Quality side. This is not necessary for documentation changes or for merge request fixing flaky failures, so in this case it's fine to skip the Danger message.

This commit updates the stable branch checks to skip the package-and-test message on trivial merge requests with documentation changes only or with fixes for flaky failures.

Related to gitlab-com/gl-infra/delivery#2839 (closed)

Screenshots

Flaky failures

Before (!112612 (comment 1287284700))	After (!113433 (comment 1299405213))
	No message (as expected)

MR with code updates (no change)

Before (!112615 (comment 1287297821))	After (!113434 (comment 1299408836))

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.