Adjust FE components for group-level MR approval rules

General flow

• Group level rules empty by default
• "Add approval rule" has two options:
  • Minimum required approval
    • Once this rule is added then it is hidden from the menu because it can only be added once.
  • Custom approval rule
• Modal of current merge request approval rule is used for adding a custom merge approval rule. (out of scope for this issue. Backend/frontend integration will be done in #425181)
  • Target branch is defaulted to "All protected branches"
  • Dropdown is disabled to prevent user editing
• Show toast when an approval is added/updated/deleted (out of scope for this issue. Backend/frontend integration will be done in #425181)
• Project level shows a popover on the lock icon for inherited rules (out of scope for this issue. It will be done in #425184)

Frontend work

1. Make sure components are in a shared directory. Rename Name to Rule. Add Minimum required approvals to Rule column and move All eligible users to Approvers column.
2. Adjust alignment for Target branch and Approvals required columns to align to the center.
3. Change actions button to tertiary icon buttons. Add screen reader only text for this column <th> element.

All iterative work will be hidden behind Feature Flags until the whole piece is completed.