Move and collapse approval rules under "reviewers" section on merge requests

Problem

With the introduction of a dedicated reviewers section on merge requests #216054 (closed), it's confusing to have an "approvers" section in addition. Since "approver" is not an official role in our model, having both approvers and reviewers becomes confusing.

Proposal

Move approval rules to be a sub-section of the "reviewers" section

Conditions	Image
Approvals optional	In this case, if approvals are not overridable, we hide the Show approval rules accordion, as there's no use in it.
Anyone can approve, X approvals required
Single rule, only some people can approve
Multiple rules

Further details

1. Approval rules on Starter tier should only allow for one rule as currently supported
2. Approval rules on Premium tier should allow multiple approval rules

Testing

• Run package-and-qa to check if the existing tests are affected.