ci: Use the trigger keyword for CNG downstream pipelines

What does this MR do and why?

Similarly to how native triggers are now used to trigger omnibus-gitlab-mirror => gitlab-qa-mirror pipelines, we're migrating the gitlab => CNG-mirror trigger to be native in this MR.

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 review-build-cng-env job for this. We split the task of variables computation and pipeline trigger to two.

In the review-build-cng-env job, we compute all the necessary information for the trigger 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.

The same thing has been done for the cloud-native-image, which depends on the cloud-native-image-env job which computes the necessary variables.

Related to omnibus-gitlab#6118 (closed).

MR acceptance checklist

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

Edited by Rémy Coutable

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