Primitives: New "mode" argument

[ptristan3]

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Describe the new content you are requesting.

We are consolidating the execution modes concept for primitives including a single parameter "mode". The incoming release will include the following:

• Deprecation of backend and session arguments for Sampler and Estimator.

• Stop supporting a backend name (string) as a valid execution mode.

• A new mode argument for the Sampler and Estimator execution modes. The mode can be a Backend, Session, Batch, or None.

• [x] Update the code examples, switching to mode parameter instead of backend and/or session.

• [ ] If there is some code example using a backend name, replace it with a backend object.

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