Update MR Approval Widget GraphQL query to use a subscription

What does this MR do and why?

For #383655 (closed)

Enables realtime updates for the MR approval widget behind the realtime_approvals feature flag.

Screenshots or screen recordings

Before	After
approval-gql-subscription-before	approval-gql-subscription-after

How to set up and validate locally

1. Create an MR that can be approved
2. In a different browser window, approve the MR
3. Note that almost immediately, the approval widget is updated with your approval
   • This does not happen on master

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.