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.

[ghost]

Thanks for contacting us.
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