Check if default branch is already protected before creating

What does this MR do?

As a part of #329422 (closed): while creating security policy project, the default branch should be protected and push_access_levels should have Gitlab::Access::NO_ACCESS. So we create protected branch for the default branch with push_access_levels.

Currently, BranchHooksService also creates a protected branch with default_branch_protection from the group that the project belongs to. This creates duplicate protected branches for the same default branch.

This change checks if the default branch is already protected before creating a protected branch. To avoid race condition from BranchHooksService, we update the branch protection for default branch if it already exists while creating security policy project.

Screenshots (strongly suggested)

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After:

Does this MR meet the acceptance criteria?

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