Enable ci_child_of_child_pipeline by default

What does this MR do?

Related to #29651 (closed) #243747 (closed)

This MR enables ci_child_of_child_pipeline feature flag by default.

And adds the changelog which was not added in the original MR because of the FF.

The feature flag has been fully enabled at 16:14 UTC.

Note: This is in WIP because I haven't enabled for 100% yet.

Screenshots

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