Fix SalesforceHook compatiblity with Pandas 2.x

[dave-pollock]

In pandas 2.0.0, pandas.np was removed.

Airflow 2.7.0 switched to pandas 2.x, however the SalesforceHook still has a reference to pandas.np, and is therefore broken in some cases after 2.7.0, resulting in an AttributeError if the _to_timestamp method is called.

This PR switches from using pandas.np to using the numpy library directly in order to fix the issue.

[boring-cyborg]

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[uranusjr]

Can a test be added to validate the fix?

[dave-pollock]

@uranusjr I've added a test case. Thanks.

[boring-cyborg]

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