Update serialization failure note to be a warn level event in `BaseEv…

…ent`

[CT-2264](#7113) wants
logging messages about event serialization failure to be `WARNING`
level events. This does that.

core/dbt/events/base_types.py

@@ -79,7 +79,7 @@ def __init__(self, *args, **kwargs):
  if "pytest" in sys.modules:
  raise Exception(error_msg)
  else:
- fire_event(Note(msg=error_msg))
+ fire_event(Note(msg=error_msg), level=EventLevel.WARN)
  self.pb_msg = msg_cls()
 
  def __setattr__(self, key, value):