Update merge request widget component to use the GraphQL subscription

Subscribe to the the mergeRequestMergeStatusUpdated GraphQL subscription that will be implemented in #372514 (closed). Should be behind a feature flag.

Testing

Add/update spec\features test to ensure approval is removed in realtime after an approvals reset action occurs (e.g. rebase with reset_approvals_on_push turned on), SET: @jay_mccure

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