Temporarily remove OptIn requirements for volatile and uncommitted API

Kotlin's opt-in requirement system is not yet stable (as of Kotlin 1.6), so we're going to remove the opt-in requirements for now.

Features annotated with VolatileCouchbaseApi and UncommittedCouchbaseApi no longer require opt-in. However, we plan to re-apply the opt-in requirements once the opt-in feature  stabilizes in a future version of Kotlin.