Use Flow Java api and drop support for reactive-streams

[hamnis]

JDK 9+ has the flow api which is a JDK supported version of reactive streams.
Maybe this library should convert to that as part of removing JDK8 support

[hamnis]

for interop with reactive-streams if needed https://www.reactive-streams.org/reactive-streams-flow-adapters-1.0.2-javadoc/org/reactivestreams/FlowAdapters.html