[master] Fix datetime.datetime.utcnow() deprecated in Python 3.12

[chloekaii]

What does this PR do?

This changes all instances of datetime.utcnow() to datetime.now(tz=timezone.utc).

What issues does this PR fix or reference?

Fixes: #65604

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

No

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[max-arnold]

I think this might be not backward compatible, because utcnow() returns a naive datetime, and now(tz=datetime.UTC) returns a timezone-aware one.

[twangboy]

To create a naive datetime

from datetime import datetime, UTC
datetime.now(UTC).replace(tzinfo=None)

[marmarek]

I think this might be not backward compatible, because utcnow() returns a naive datetime, and now(tz=datetime.UTC) returns a timezone-aware one.

That's the point of this deprecation warning, no? To stop using non-timezone-aware objects.

[marmarek]

This doesn't work: TypeError: can't subtract offset-naive and offset-aware datetimes
A better approach is to use astimezone() instead of calculating timezone_delta and adding it manually:

local_start_time = utc_start_time.astimezone()
local_finish_time = utc_finish_time.astimezone()

[twangboy]

This needs to be rebased and there are quite a number of test failures. This will also need a changelog.
Some of these changes are in @marmarek's PR #66902