Switch `ZonedDateTime` to `OffsetDateTime`

[pwall-tyro]

The cloudevents spec clearly says that the time attribute "MUST adhere to the format specified in RFC 3339". The java.time.xxx class that corresponds to RFC 3339 is OffsetDateTime.

ZonedDateTime adds a time zone identifier (in the form of region/city) to the end of the string on serialization, meaning that an event containing a time output by normal serialization does not conform the the spec.

OffsetDateTime is easier to work with, takes less space and more correctly implements the spec.

[slinkydeveloper]

Hi @pwall-tyro, thanks for catching up this issue, i'm on it