setTaskScheduler in StompBrokerRelayRegistration breaks chaining

[rstoyanchev]

The recently added setTaskScheduler returns void instead of StompBrokerRelayRegistration and so does setTcpClient which was added longer ago might be worth addressing.

Originally reported in #22822 (comment).

[jhoeller]

Let's address both in 5.3.1 indeed. This is still early enough, in particular since the change is source compatible (just not binary compatible), so fresh application builds won't see any regression.