Add post stage to trigger code sync

Related issue: https://jihulab.com/gitlab-cn/gitlab/-/issues/2715

What does this MR do and why?

Add a new .post stage job to trigger code sync pipeline, to merge pre-main-jh into main-jh,

only be triggered on pre-main-jh branch successful pipeline.

Screenshots or screen recordings

Non-UI

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.