fix: resolve Python Datetime warnings

[emmanuel-ferdman]

Summary

This small PR resolves the datetime library warnings:

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). or datetime.datetime.utcnow()

DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).

Note that .replace(tzinfo=None) allows to keep the original behavior where the time appears as a naive UTC timestamp (i.e., without any timezone offset).

[emmanuel-ferdman]

@willydouhard could you please review this small PR when you get the chance? thanks 😄

[hayescode]

@emmanuel-ferdman I'm just going to move the datetime helpers from literalai to chainlit and handle this all there.