feat!: deprecate the time __unit__ parameter

[agoose77]

In future we anticipate the introduction of a units system that uses this parameter. Moreover, datetimes should use the primitive to represent these types, as it's a fundamental type.

[codecov]

Codecov Report

Merging #2518 (0e0e584) into main (645c817) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/forms/numpyform.py	96.63% <100.00%> (+0.14%)	⬆️

[jpivarski]

This looks great!

All of the tests that assert the DeprecationWarnings will have to be changed later, but it will be obvious when the time comes.