Code and content updates for the latest Qiskit Runtime changes

[beckykd]

URL to the relevant guide or resource

Will have to check all of the docs.

Select all that apply

• new content request
• typo
• code bug
• out-of-date content
• broken link
• other

Describe the fix or the content request.

• A new channel type, ibm_quantum_platform, has been introduced for service initialization and is the new default. (When activating the service, the channel must be set to ibm_cloud or ibm_quantum_platform. Both of these channels point to the new IBM Quantum Platform API but the ibm_cloud channel will be deprecated and removed in a future release.)
• An instance value is no longer required for saving or
  initializing an account on the new platform. If an instance is not passed in, all instances will be checked when a backend is retrieved. If an instance is saved or passed in
  during initialization, it will be used as the default instance when retrieving backends.
  The instance passed in at initialization will take precedence over the one saved in the account.
• You can specify an instance when setting the backend.
• You can specify the default region to use and the types of plans to prioritize when letting the service choose the instance to use.
• You can specify the instance by CRN or instance name. (for QiskitRuntimeService() and save_account() )
• There is a new property that returns whether or not a job is private.

@kt474 is updating the migration guide, so we can get a lot of content from there.

For new content requests - if the request is accepted, do you want to write the content?

I will write (or already have written) a draft of the proposed content

[beckykd]

@kt474 - ready for review!