Fix DeprecationWarning: datetime.datetime.utcnow()

[Jeff Schaller]

Is your feature request related to a problem? Please describe.

Builds sometimes output a deprecation warning; for example: https://github.com/Charcoal-SE/SmokeDetector/actions/runs/6731552476/job/18296462660#step:10:65

Describe the solution you'd like

It looks like there's a suggested alternative: "Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC)."

Describe alternatives you've considered

No other alternatives at this time.

Additional context

globalvars.py:84
/home/runner/work/SmokeDetector/SmokeDetector/globalvars.py:84: 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).
startup_utc_date = datetime.utcnow()

[Jeff Schaller]

It looks like the fix consists of:

1. changing from datetime import datetime to from datetime import datetime, timezone
2. changing all instances of datetime.utcnow() to datetime.now(tz=timezone.utc)

Currently working on this on a branch, and will submit a PR when it's ready