Start preparation job before merging backports

What does this MR do and why?

Describe in detail what your merge request does and why.

Content

• Start preparation job before merging backports

In the current process, the preparation step is done before merging backports. Thus, this commit reflect it to the pipeline.

Ref: gitlab-com/gl-infra/delivery#20190 (closed)

Testing

Starting a new pipeline from this branch with SECURITY_RELEASE_PIPELINE=true . As you can see, the preparation step now happens in between early_merge and backport_merge.