[Feature flag] Rollout of `project_approval_rule_all_protected_branches`

What does this MR do and why?

DO NOT MERGE UNTIL 2022-08-01 09:00 UTC AT THE EARLIEST TO GIVE TIME FOR VERIFICATION

This MR removes the feature flag project_approval_rule_all_protected_branches and updates the frontend to enable the feature for project MR approval rules without the need for the rails helper data.

It also makes sure the documentation is up-to-date.

Screenshots or screen recordings

Screen_Recording_2022-07-19_at_14.42.05

How to set up and validate locally

The UI uses the API, so by testing the UI we get to test everything at the same time 😄

  1. Visit a projects settings page and open the Merge request approvals section e.g. http://gdk.test:3000/gitlab-org/gitlab-test/edit#js-merge-request-approval-settings.
  2. Create or edit an approval rule and test that you can select all branches, all protected branches and a specific branch by selection or search.
  3. Test that setting all branches or all protected branches and saving the approval rule shows the correct value on the rules list.
  4. Create a new MR and test that the approval rule with all branches or all protected branches is picked up.
  5. Create a new MR targeting a non-protected branch and check that the all protected branches rule is not picked up.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #364091 (closed)

Edited by Robert Hunt