Update start notebook to use `get_engine` instead of `get_qcs_objects_for_notebook`

[senecameeks]

Addresses #6427 by removing the incorrect processor_id in the notebook.

Although this doesn't fix the underlying issue as to why get_qcs_objects_for_notebook is incorrect and doesn't address Will's second point in his comment

FYI, I verified this works as a good replacement for get_qcs_objects_for_notebook

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All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.76%. Comparing base (d75d43f) to head (02d998b). Report is 1 commits behind head on main.

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```diff @@ Coverage Diff @@ ## main #6483 +/- ## ======================================= Coverage 97.76% 97.76% ======================================= Files 1105 1105 Lines 94936 94963 +27 ======================================= + Hits 92810 92837 +27 Misses 2126 2126 ```

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[NoureldinYosri]

this might not work if it's run from outside corp network in which case authentication is needed which is part of get_qcs_objects_for_notebook https://github.com/quantumlib/Cirq/blob/eef7c5c9309fcdb9254a295491facee3118c2279/cirq-google/cirq_google/engine/qcs_notebook.py#L104

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to check if it will from outside corp network try to run then notebook in colab.research

[senecameeks]

Thanks @NoureldinYosri for the catch!

6485 exposes authenticate_user, which I've added to this start notebook.

In the case where get_engine throws an exception, it will default to a virtual engine instance using get_qcs_notebook_objects. Note that because virtual is set to True, it does not print the erroneous message Using processor: which was the impetus for this change (See #6427).