Provide more info about how to migrate to backendV2 in api ref deprecation warnings

[javabster]

We've received user feedback that this deprecation notice pops up a lot in the API ref but it's a bit frustrating because it just tells people they need to upgrade to backendV2 but doesn't provide info on how. Is there somewhere we could link users to in this message to help them get more information on migrating to backendV2?

[mtreinish]

For the most part BackendV2 is not something for an end user to migrate to as a user typically doesn't implement the backend interface. It's something that a provider author will need to migrate to which is a small subset of Qiskit users.

That being said the guide on how to adjust user access patterns from BackendV1 -> BackendV2 is probably what is missing here: https://docs.quantum.ibm.com/api/qiskit/providers#migrating-from-backendv1-to-backendv2 and I suggest we cross ref to it in the deprecation note.