Disallow standard calendar np.datetime64 encoding prior to reform

[spencerkclark]

In #9618 we allowed encoding np.datetime64 values prior to 1582-10-15 using a "standard" or "gregorian" calendar through cftime. While technically possible, this implicitly introduces a calendar change, and means the values can no longer be round tripped as np.datetime64—xarray will choose cftime.DatetimeGregorian instances when decoding instead. I am not sure how often this will come up and the behavior may not be the user's intent.

This PR switches to raising a ValueError in this circumstance, and recommends encoding with a "proleptic_gregorian" calendar instead (the calendar that xarray automatically chooses for np.datetime64 values if provided no user input).

cc: @kmuehlbauer (this is one possible way to address the issue in #8324)

• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

[kmuehlbauer]

@spencerkclark +1 to have the user decide.

[spencerkclark]

Sounds good—thanks @kmuehlbauer—I'll merge this in so we can move forward with #8324.