Use kotlin.time.Duration instead of java.time.Duration

In Kotlin 1.6 kotlin.time.Duration is no longer marked as Experimental, so we can go ahead and use Kotlin's Duration in the SDK API.

This is nice because instead of `Duration.ofSeconds(30)` you can write `30.seconds`.

Breaking Changes:

Yeah, this breaks everything that used java.time.Duration.
Also, the following methods have been deprecated:

• Expiry.ofSeconds
• Expiry.ofMinutes
• Expiry.ofHours
• Expiry.ofDays

Instead, use `Expiry.of(x.seconds)` (or minutes, hours, days)