Branch rule editing: Minimum required approvals per target branch
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Add an option to create _Minimum required approvals_ rule per target branch, when on Branch rule details page:
- Update the labelling of "All eligible users" as "Approvers" and make the rule name "Minimum required approvals"
- make sure created rule has a target branch of a branch you're visiting the Branch rule details page, even though we do not display it in the table
- Update the UI in "Settings \> Merge requests \> Merge request approvals" to reflect the minimum required approvals for target branches (probably will show without any changes)
- there is no edit action for this rule, it's possible to be edited from the table row (only the input for _Approvals required_). So in this sense it behaves like an empty rule, but for a certain branch

Figma: https://www.figma.com/design/eq2nf3B8MhB9xKrC0VzbPf/Repository?node-id=916-8402&t=CA8bNxlsO81Eiu8S-4
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