Replace deprecated `utcnow()` calls with `now(timezone.utc)`

[shreddd]

Fix to address deprecated datetime.utcnow() calls

[eecavanna]

I confirmed that, even in the Python 3.10 docs, this replacement is "recommended."

The reason I checked that version of the Python docs is that that is the Python version that is running in the container image that runs the FastAPI app, according to its Dockerfile.

[eecavanna]

I'm comfortable with this branch being merged into main as is.