Trigger keyword works on free EE but not free CE, preventing use of child-parent pipelines

Summary

It is not possible when using GitLab CE to use parent-child pipelines because this requires the trigger keyword, which is not available in CE.

This is related to the issue #31573 (closed) which resolved some other issues that touched on CE/EE split, but did not address changes to trigger since that was considered syntactic sugar at the time. It appears though we've release child/parent pipelines only behind the trigger keyword, with no actual (or at least documented) way to trigger them, exacerbating this continued split.

Steps to reproduce

Use the trigger keyword on a free CE project. See that it does not work (no trigger keyword is available). Try again on EE project at free tier and see it work.

Example Project

TBD

What is the current bug behavior?

trigger keyword is unavailable.

What is the expected correct behavior?

trigger keyword is available.

Relevant logs and/or screenshots

N/A

Output of checks

This bug happens on gitlab.com.

Possible fixes

At this point I believe it's best to make the trigger keyword available in core. We have a stewardship promise to not move features up, and anyway this is only providing a very small bit of value but giving an option to use a native keyword instead of a curl to the API. We will continue to differentiate the multi-project pipelines feature through dashboards and multi-project views rather than preventing ease of use in the .gitlab-ci.yml.

Documentation will need to be updated as it relates to the trigger keyword.