Protected branch is now created for default branch on import

Before this change when we imported a project we would have a unprotected branch which was not the intended behaviour.

This MR makes it so that on import the project automatically turns its default branch into a protected branch.

Does this MR meet the acceptance criteria?

• EE Compat check fixed
• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides

What are the relevant issue numbers?

Closes #3968 (closed)