Deprecate passing a string for backend

[jyu00]

What is the expected feature or enhancement?

Related to #1681.

Today we allow one to pass the backend name (string) when initializing Sampler/Estimator/Session/Batch, for example, you can do

sampler = SamplerV2(backend="ibm_osaka")

This would actually call QiskitRuntimeService() (i.e. use your default account setting) under the cover, which can be a surprise to people if the default channel or h/g/p is not what they want to use.

Furthermore, some people would do

service = QiskitRuntimeService(name="foo")
sampler = SamplerV2(backend="ibm_osaka")

In the past this would cause confusion because it would actually use the default account instead of the account named foo. To address this, we added code to use the last-initialize account instead of the default one. But this was also inferring user's intent and contradicts with the docstring which says If a name is specified, the default account is used.

To make the account selection more explicit, we can 1) make both service and backend required or 2) disallow passing a string for backend.

With the ISA circuit requirement, people are more likely to initialize a backend object now to do transpilation, so (2) seems to be the better choice.

Acceptance criteria

• Deprecate passing a string for backend when initializing Session, Batch, SamplerV2 and EstimatorV2.