Remove references to cross_project_pipeline source

What does this MR do?

In #195991 we were planning to rename the pipeline source when triggered as multi-project pipeline from pipeline to cross_project_pipeline in %13.0.

We had documented that but we didn't do it in last major release.

A community member highlighted that and we need to remove this reference from the docs until we decide whether it's going to happen in %14.0 or ever.

Screenshots (strongly suggested)

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