Use native triggers for running QA test job

What does this MR do?

Use native triggers for multi-project pipelines instead of custom scripts. We're starting with moving omnibus-gitlab-mirror => gitlab-qa-mirror trigger.

Because trigger/bridge jobs doesn't have any script capability (they are executed by sidekiq), any computations that needs to be done to calculate the variables that are to be passed to downstream pipeline needs to be done elsewhere. We use generate-facts job for this. We split the task of fact computation and pipeline trigger to two. In the generate-facts job, we compute all the necessary information for all the triggers (for now, QA only) and make the output a dotenv report, which is received by trigger jobs. And then, the trigger jobs pick and pass the required variables to the downstream pipeline.

Related issues

Related to #6118 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened