Normalize offset strings to UTC when casting to naive types

[youdie006]

#4775 normalized a non-UTC %DateTime{} to UTC for :date, :naive_datetime and :naive_datetime_usec, and #4792 did the same for :time. An ISO 8601 string carrying the same offset still goes through NaiveDateTime.from_iso8601, which drops the offset, so one instant casts two ways:

input	:naive_datetime	:date	:utc_datetime
"2020-06-01T00:30:07+02:00"	~N[2020-06-01 00:30:07]	~D[2020-06-01]	~U[2020-05-31 22:30:07Z]
the same instant as %DateTime{}	~N[2020-05-31 22:30:07]	~D[2020-05-31]	~U[2020-05-31 22:30:07Z]

This parses with DateTime.from_iso8601 first and falls back to NaiveDateTime.from_iso8601, so strings without an offset, including Phoenix datetime-local values, are unchanged. It follows the conclusion in #2054 that a dropped offset should either be converted or rejected. If you would rather reject non-zero offsets for the naive types, that is a small change and I can switch to it.

Verification

Base f4d84a1a. New rows go into the existing :date, :naive_datetime and :naive_datetime_usec cast tests, mirroring the :utc_datetime offset rows at test/ecto/type_test.exs:1006 and :1011.

row	lib/ecto/type.ex md5	test/ecto/type_test.exs
master	e91eacf61b	3 failures: the new date, naive and naive_usec rows
this PR	27ba9a1702	0 failures
date site left as before	43d4775fc6	1 failure: the date row
require an offset	9957058100	5 failures, all existing rows such as cast(:date, "2015-12-31T00:00:00")

The CI unit-test steps on Elixir 1.18 / OTP 27: mix deps.unlock --check-unused, mix compile --warnings-as-errors, mix test (97 doctests, 1512 tests, 0 failures); mix format --check-formatted on both files. The same on Elixir 1.14.5 / OTP 26 passes. Not run: the exact OTP pins of the matrix and the Earthly integration job.

Written with AI assistance (Claude); the measurements above were run locally and I have reviewed the change.

[josevalim]

Thank you. Upon further inspection, I am reverting the other two PRs, because the dropping of offsets is exactly how Elixir behaves for NaiveDateTime, Time, Date when they pass a DateTime. In other words, we respect the wall time of said datetimes. Which makes sense, if you ask Date.days_in_month, you want the current datetime, not the UTC variant.