Use `trigger` keyword for multi-project pipelines instead of custom script

Why we can't use trigger keyword

Listing why we currently don't use trigger keyword for multi-project pipelines but use a custom script. Incomplete list - will expand as I remember the reasons.

Trigger from gitlab to omnibus-gitlab-mirror

1. Contents of *_VERSION files can't be passed as trigger variables.
2. Some variables have different values based on if conditionals in the trigger-build script. Need to check if they can be rewritten using variables.

Trigger from omnibus-gitlab to omnibus-gitlab-mirror

1. Trigger jobs won't have any before/after script. This means we will have to add a job to check whether the commit we want to run a pipeline against is present in the mirror.
   • NOTE: This is not a big issue, because we won't be adding any new job but replacing the existing one with a different one. (Trigger jobs are executed in Sidekiq and won't take up CI resources)

Trigger from omnibus-gitlab-mirror to gitlab-qa-mirror

1. The variable RELEASE is computed by a Ruby method and can't be known at the time of CI yaml evaluation.

What added functionality we have with custom script

1. A comment with link to downstream pipelines is posted to the commit as a comment (which will show up in MRs also) - this helps a lot in easier navigation.