Branch rule editing: Minimum required approvals per target branch

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