Upgrade to V7 of RabbitMQ Client

[iancooper]

Is your feature request related to a problem? Please describe.
RabbitMQ has released version 7, which has breaking changes. This means it is not a simple dependency upgrade, but a code fix. For this reason, raising a ticket to track it.

Describe the solution you'd like
There is a migration guide at: https://github.com/rabbitmq/rabbitmq-dotnet-client/blob/main/v7-MIGRATION.md

Describe alternatives you've considered
We could chose to remain on 6.X.X for longer, but as we encapsulate the RMQ dependency for our users, there is no real benefit to doing so

Additional context
Possible difficulties include async code and our own event consumer

[iancooper]

@lillo42 I suggest that we make this change for V10 only, following completion of the work in #3418. I'll get the work finished soon, then we can merge and get this in for V10

[iancooper]

This means overall our strategy is: V9 => V6 RMQ, V10 => V7 RMQ. Once we see the performance implications for RMQ on V10 (mainly significant around expectations for the Reactor as opposed to Proactor pipeline).