patched GetCurrentTime to use correct timezone-aware datetime

[b-d-e]

Addresses #19161 by addressing the deprecation warning.

See python 3.12 release notes for motivating python change:

datetime: datetime.datetime’s utcnow() and utcfromtimestamp() are deprecated and will be removed in a future version. Instead, use timezone-aware objects to represent datetimes in UTC: respectively, call now() and fromtimestamp() with the tz parameter set to datetime.UTC. (Contributed by Paul Ganssle in gh-103857.)

Existing unit tests should handle these changes.

Without this, this warning appears when using protobuf:

.venv/lib/python3.12/site-packages/google/protobuf/internal/well_known_types.py:178: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
  self.FromDatetime(datetime.datetime.utcnow())

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[b-d-e]

This won't work as is under Python 3.8.x, but that version has passed end-of-life support from Python - I don't know what the intended future support from protobuf is?

If desired, I can add a conditional check that changes the behaviour based on if version <3.9, using the old method if so.

[JasonLunn]

@b-d-e - Can you take a look at the failing tests?

[b-d-e]

@JasonLunn - as far as I can see the test is failing with:

This pull request is from an unsafe fork and hasn't been approved to run tests.
A protobuf team member will need to review the PR and add the 'safe for tests' tag.

I can see the safe for tests tag was added a while back, but I can't see the output if any actions that were triggered by that? (sorry if I am missing something obvious here)