Migrate away from Joda-Time to java.time #144

Summary

Migrate killbill-client-java from Joda-Time to the java.time (JSR-310) API.

Joda-Time has been in maintenance mode since Java SE 8 introduced java.time. The project itself recommends migrating to java.time.

Tasks

• Identify all usages of Joda-Time (org.joda.time) across the codebase
• Replace DateTime with java.time.OffsetDateTime or java.time.ZonedDateTime
• Replace LocalDate with java.time.LocalDate
• Replace DateTimeZone with java.time.ZoneId
• Update serialization/deserialization (Jackson, JSON mappings)
• Remove Joda-Time dependency from pom.xml
• Update tests
• Ensure backward compatibility of the public API (or document breaking changes)