Remove deprecated Observable (since Java 9)

[ghusta]

See :

• https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Observable.html
• https://stackoverflow.com/questions/46380073/observer-is-deprecated-in-java-9-what-should-we-use-instead-of-it
• https://www.reactive-streams.org/
• https://medium.com/@marilyn_2414/reactive-programming-with-jdk-9-flow-api-processor-implementation-8c572c827132

The Observer pattern is push, while the Flow API is pull.

Alternatives :

• https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/concurrent/Flow.html
• https://reactivex.io/RxJava/3.x/javadoc/io/reactivex/rxjava3/core/Observable.html