Fix/update default branch after importing by gitlab custom project template

What does this MR do and why?

Describe in detail what your merge request does and why.

Fix #352811

Screenshots or screen recordings

How to set up and validate locally

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

1. Create a group A
2. Create a subgroup B for A
3. Set B to the Custom project templates
4. Create a project P1 for B
5. Create a new branch B1 for P1 and change the default branch to B1
6. Create a new project P2 using custom project template P1
7. The default branch of P2 is main which should be B1

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.