Add ability to have zero approvers

What does this MR do?

• Add ability to have zero approvers
• Approvals section in MR settings is available at all times

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

To increase approvals usage, remove blockers from the initial onboarding flow by keeping the required approvals at zero.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides

What are the relevant issue numbers?

#2452- Always on approvals