Port `trigger` keyword to Core (4/4) - move trigger syntax

What does this MR do?

Related to: #199224 (closed)

This is part 4 of 4 where we goal is to move the trigger keyword from ee/ to Core. None of these changes introduces new behavior. We are moving existing behavior which was available for free on EE installations to Core.

In this MR we are moving the trigger syntax to Core and changing the documentation. This is the largest out of the 4 parts because by moving the trigger config entry we also need to move the bridge and make changes to the jobs entry.

• In part 1 we moved Ci::Bridge model to Core.
• In part 2 we moved the interaction between Ci::Pipeline and Ci::Bridge for behavior related to updating bridge status that connects 2 pipelines.
• In part 3 we moved the downstream pipeline creation logic to Core when a Ci::Bridge moved to :pending.

Full port was originally implemented in !24191 (closed) but I'm splitting it into 4 MRs due being huge.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team