Fix error when third level trigger pipeline

What does this MR do?

When implementing child of child pipelines (!41102 (merged)), we added not only the parent-child pipelines but also "accidentally" the triggered pipelines, which have different projects, into the hierarchy tree.

That caused this error:

• Project A has Pipeline 1
• Pipeline 1 triggers Pipeline 2 of Project B
• Pipeline 2 triggers Pipeline 3 of Project C
• Pipeline 3 tries to create a child pipeline of Project C: Pipeline 4
• Pipeline 4 should be created successfully, but it can not be because our newly introduced hierarchy tree assumes that this child has 3 level depth in the same family!.

gitlab-ci.yml for Project A

build:
    stage: build
    script: exit 0

trigger_next:
    stage: deploy
    trigger:
        project: project-b

gitlab-ci.yml for Project B

build:
    stage: build
    script: exit 0

trigger_next:
    stage: deploy
    trigger:
        project: project-c

gitlab-ci.yml for Project C

build:
    script: exit 0

deploy:
    trigger:
        include: child.yml

child.yml

build_child:
    script: exit 0

In this MR, we are starting to ignore other project pipelines.

• Related to: gitlab-com/gl-infra/production#2700 (closed)
• Related to: #29651 (closed) !41102 (merged)

Screenshots

Before the fix

After the fix

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