fix(aws_s3 source): Correct serde tagging on event enum (#14686)

A change was made to the aws_s3 source to add support for test events, which are
sent when notification configuration is changed. However, the new enum
representing both event types was left to the default representation, which is
"externally tagged" which does not actually match the encoding of the events.
This PR corrects that deserialization policy to "untagged", as the incoming
events are not tagged.