Use ZonedDateTime for ISO-8601 timestamp types

The ZonedDateTime type was introduced in Java 8 and is functionally equivalent to the Joda DateTime type. Joda itself recommends using the java.time types starting with Java SE 8.

It would be preferable to depend on standard Java implementations for handling ISO-8601 types, as modern Java applications are more and more depending on ZonedDateTime rather than joda libraries for time-related types. Java SE 7 has been end-of-life since April 2015, so I believe backward compatibility should not be a relevant concern anymore.

Example code which uses Joda DateTime:
`azure.activityLogs().defineQuery().startingFrom().endsBefore()`

The startingFrom() and endsBefore() methods accept a Joda DateTime as an argument.