Move and collapse approval rules under "reviewers" section on merge requests
## Problem
With the introduction of a dedicated reviewers section on merge requests https://gitlab.com/gitlab-org/gitlab/-/issues/216054, 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 | <br>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.
issue