Enable approval rules drawer feature flag by default + update docs

What does this MR do and why?

Enables the approval_rules_drawer feature flag by default, and adds a note to the docs about the change.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

Branch rules

1. Add a Branch rule
   • Navigate to Project > Repository > Settings > Branch rules > Add branch rule
2. View the branch rule created in step 1
   • Navigate to Project > Repository > Settings > Branch rules > View details
3. On the Branch rule details screen the Approval rules app should display and work as expected
4. Edit/Add should should a drawer containing the approval rule form

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Settings > Merge request approvals

1. Navigate to Project > Settings > Merge requests > Merge request approvals
2. Adding, editing, deleting approval rules should work as expected

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Merge request Add/Edit

1. Navigate to Merge request > New or Merge request > Edit
2. Adding, editing, deleting approval rules should work as expected

Related to #444628 (closed)