Fix deprecation warnings for `datetime.utcnow()`

[tomasr8]

This shows up when running the tests on 3.12:

~dev/babel/tests/test_dates.py:757: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to rep
resent datetimes in UTC: datetime.datetime.now(datetime.UTC).                                                                                                                                 
    assert dates.format_datetime(datetime.utcnow(), locale="ja_JP@mod")

I think there's no harm in using plain datetime.now() for these tests.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.22%. Comparing base (cf7d223) to head (0969ad5). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1119 +/- ## ======================================= Coverage 91.22% 91.22% ======================================= Files 27 27 Lines 4561 4561 ======================================= Hits 4161 4161 Misses 400 400 ``` | [Flag](https://app.codecov.io/gh/python-babel/babel/pull/1119/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | Coverage Δ | | |---|---|---| | [macos-12-3.10](https://app.codecov.io/gh/python-babel/babel/pull/1119/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.02% <ø> (ø)` | | | [macos-12-3.11](https://app.codecov.io/gh/python-babel/babel/pull/1119/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `89.95% <ø> (ø)` | | | 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