Drop support for RxJava 1 and 2

See https://github.com/spring-projects/spring-framework/issues/27443

Tickets from other modules:

* Cassandra https://github.com/spring-projects/spring-data-cassandra/issues/1173
* MongoDB https://github.com/spring-projects/spring-data-mongodb/issues/3839