Update Kotlin version to use Duration?

[Derek Peirce]

Kotlin has a Duration type that would be useful to include for the various time-based Rx operators. (For example, using single.timeout(5.seconds) instead of single.timeout(5, TimeUnit.SECONDS).) However, this version of Duration was not finalized until 1.6 to be based on Long. RxKotlin is currently on Kotlin 1.4, so Duration is instead entirely different, using a Double instead. Can we increase the Kotlin version to 1.6 or further so that Duration is properly supported?

I can add the extension methods myself, I just need to first clear that the repo owners would be alright with increasing the Kotlin version as well, or else it won't be useful.