fix(ndb): replace deprecated datetime.utcnow() and utcfromtimestamp()

[skippdot]

Fixes #15840

datetime.datetime.utcnow() and utcfromtimestamp() are deprecated since Python 3.12 and scheduled for removal. In google-cloud-ndb they are called from the _now() staticmethods of DateTimeProperty, DateProperty and TimeProperty, so every auto_now / auto_now_add put currently emits a DeprecationWarning; the legacy value decoder emits another one for its epoch constant. Running the unit suite on Python 3.14 produced 70 such warnings.

Changes

• Add a private _utcnow() helper to model.py, next to _getfullargspec, and route the three _now() methods through it.
• Build the _EPOCH constant in _legacy_db_get_value() as datetime.datetime(1970, 1, 1) instead of utcfromtimestamp(0) (same value).
• Update the tests that used the deprecated calls; add test__utcnow.

Why the helper strips tzinfo

The recommended replacement, datetime.now(timezone.utc), returns an offset-aware value, but DateTimeProperty._validate() rejects offset-aware values unless the property is configured with tzinfo. A plain substitution would therefore make auto_now raise BadValueError for every property without tzinfo. The helper calls .replace(tzinfo=None) to keep returning the naive-UTC value ndb has always stored and compared — the same approach google-auth takes in _helpers.utcnow().

Verification

• nox -s unit-equivalent run on Python 3.10 and 3.14: 1835 passed.
• utcnow / utcfromtimestamp warnings: 70 → 0.
• ruff format --check and flake8 clean.
• System tests collect (360); the touched test_insert_datetime_property_with_tz now uses the same instant for both the aware and the naive value.

[gemini-code-assist]

Code Review

This pull request replaces deprecated datetime.datetime.utcnow() and datetime.datetime.utcfromtimestamp() calls with Python 3.12-compatible alternatives. A new helper function _utcnow() is introduced to return naive UTC datetimes, preserving the expected behavior of ndb which stores and compares naive datetimes. The changes are accompanied by updated unit and system tests. There are no review comments to address, and the implementation looks solid.