Remove ibm_quantum channel to support runtime 0.41+

[ElePT]

Summary

After the sunset of the IQP classic platform, this PR implements two important changes in the serverless clients:

1. Stops accepting ibm_quantum as a valid input channel
2. Updates the default channel to ibm_quantum_platform instead of ibm_quantum

As a consequence:

3. The instance input argument will always be required from now on, as the argument was only optional with the ibm_quantum channel.

These two changes will allow the client to be compatible with qiskit-ibm-runtime 0.41 and onwards, fixing #1702.

It's important to note that Channels.IBM_QUANTUM is still present and no changes have been performed on the gateway or API to allow for a gradual transition for partner function users and developers. Users can update to the new client to use the latest runtime version without any breaking changes to the server-side code, and allow extra time for function developers to transition away from any unexpected dependencies on the ibm_quantum channel before its complete removal from the API.

Details and comments

This PR must be accompanied by a PR on qiskit-ibm-catalog to update the default channel value on the catalog clients.

See: Qiskit/qiskit-ibm-catalog#60

[Tansito]

Thanks @ElePT ! I see everything fine, I only have one comment related with the IBM Quantum channel. Should we just remove it from the client enum also? It doesn't affect to the gateway I think and this logic it's only used in the client so I think it's safe to remove it.

[ElePT]

I have applied your suggestion and also modified the pin on the qiskit-ibm-runtime package to >=0.40.1, <1. 0.40.1 is the first working version post-IQP-migration that I think makes sense to support for consistency with the channels supported in this serverless version.

[Tansito]

Thanks @ElePT LGTM! 😄