update datetime.utcnow() to use UTC timezone-aware object

datetime.utcnow() is being deprecated in Python 3.12 as it returns a naive
datetime object without timezone information. Replace with datetime.now(UTC)
to use an explicit timezone-aware object, preventing potential timezone
ambiguity issues.

In access_token.py, updated the token expiration calculation to use the
recommended timezone-aware approach.

livekit-api/livekit/api/access_token.py

@@ -154,9 +154,9 @@ def to_jwt(self) -> str:
             {
                 "sub": self.identity,
                 "iss": self.api_key,
-                "nbf": calendar.timegm(datetime.datetime.utcnow().utctimetuple()),
+                "nbf": calendar.timegm(datetime.datetime.now(datetime.UTC).utctimetuple()),
                 "exp": calendar.timegm(
-                    (datetime.datetime.utcnow() + self.ttl).utctimetuple()
+                    (datetime.datetime.now(datetime.UTC) + self.ttl).utctimetuple()
                 ),
             }
         )