Remove support for RxJava 1 and 2

[jhoeller]

Following up on #19628, we're finally removing RxJava 1.x support from Spring's ReactiveAdapterRegistry since it has been superseded not only by RxJava 2.x but even RxJava 3.x in the meantime.

As per a later note, we're also removing RxJava 2.x support right away since RxJava 2 reached EOL itself in February 2021 already.

[jhoeller]

Reopening for removing RxJava 2.x support - which reached EOL in February 2021 - as well, since it'll be 1.5 years by the time we go GA.