Replace the dropdown selection for default branch protection with a form that matches protected branches

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Problem to solve

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The 4 options that are currently available for default branch protections do not cover all the variations of the protected branch.

Proposal

Replace the form-select with a form-group that matches protected branch options.

Admin area

Not protected	Protected (Default)

Group

Allow owners to make changes	Only administrators can make a change
Admins always have control	Disable the group

References

Open Figma →

This page may contain information related to upcoming products, features and functionality. It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes. Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.

Implementation plan

frontend weight estimated at 2️⃣

1. Update app/views/shared/_default_branch_protection.html.haml to show the new options.
2. Update specs.

backend complete 8️⃣

It looks like there has recently been new work done to allow these type of protected branch settings at the group level(!107792 (merged))

I am currently thinking that this change will be broken in 2 phases

1. refactor the existing code to use the ProtectedBranch model rather than the hardcoded settings in access.rb 5️⃣
2. Once the existing code is using ProtectedBranch, support the new options via that model 3️⃣

I am still trying to wrap my head around how application settings generally, and protected branches specifically, are implemented. It is possible that only phase 1 is needed, and we'd get phase 2 automatically, making the overall effort a 5️⃣. Erring on the side of a higher estimate for the time being

documentation

1. Update the documentation for default branch protection.