refactor: Default to UTC when parsing dates without a known timezone (#…

…2619)

singer_sdk/streams/core.py

@@ -378,7 +378,7 @@ def _parse_datetime(self, value: str) -> datetime.datetime:  # noqa: PLR6301
 
         # Ensure datetime is timezone-aware
         if not result.tzinfo:
-            result = result.astimezone()
+            result = result.replace(tzinfo=datetime.timezone.utc)
 
         return result