Process at most 4 pipelines during push

What does this MR do?

This limits the number of pipelines that we create in a single push to 4.

This is to prevent doing git push --all or git push --mirror to create a ton of Pipelines.

This does not prevent every possible abusive scenario, but rather prevent mistakes when dealing with git repository.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/51401

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Style guides

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