Use branches bulk creation for Pull Mirroring

What does this MR do and why?

Contributes to #368797 (closed)

Feature flag is globally enabled since Aug 3, 2022.

Problem

Creating branches one-by-one is not efficient and affects the performance.

Solution

Use Branches::CreateService#bulk_create method to add new branches.

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.