Update Java SignalR client to use RxJava3

[Bludator]

Java SignalR client still uses RxJava2.

RxJava2 reached end-of-life last month so it is beyond time to update to RxJava3.

[BrennanConroy]

Makes sense, we'll need to update. In the meantime, I believe you can add RxJava3 to your project manually and it'll use that version.

Thanks for contacting us.
We're moving this issue to the Next sprint planning milestone for future evaluation / consideration. We will evaluate the request when we are planning the work for the next milestone. To learn more about what to expect next and how this issue will be handled you can read more about our triage process here.