[15.9] ci: Fix omnibus trigger target branch for MR targeting stable branches

What does this MR do and why?

Backport of !119741 (merged) to 15-9-stable-ee.

This fixes the omnibus trigger target branch for MRs targeting stable branches, in addition to support for stable branch pipelines that was brought by !119363 (merged).

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.