Add protected branches import validation for Github importer

What does this MR do and why?

Update spec to validate protected branches import.

Closes: #370842 (closed)

How to set up and validate locally

cd qa && bundle exec rspec qa/specs/features/api/1_manage/import_github_repo_spec.rb

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.