ci: Use MR target branch directly for omnibus package trigger (19-3-stable)

The earlier fix (#7534 (closed)) set build-package-and-qa's omnibus branch via an intermediate OMNIBUS_BRANCH variable. A nested variable is not expanded in trigger:branch, so downstream pipeline creation failed with "Reference not found".

Reference the predefined $CI_MERGE_REQUEST_TARGET_BRANCH_NAME directly in trigger:branch, and require a merge request in both rules so the target branch is always set. A stable-branch MR then packages on the matching omnibus stable branch and master MRs on master.

#7534 (closed)

Changelog: fixed