[JENKINS-51541,JENKINS-51551,...] - Remoting 3.21 + Allow passing custom...

[JENKINS-51541,JENKINS-51551,...] - Remoting 3.21 + Allow passing custom CommandTransport implementations to SlaveComputer from ComputerLauncher (#3455)

* [JENKINS-51541] - Introduce new SlaveComputer#setChannel() method which takes custom ChannelBuilder and CommandTransport

* [JENKINS-51551] - Pick Remoting version with the API patch

* [JENKINS-51541] - Listener is nullable according to the documentation

* [JENKINS-51551,JENKINS-51223,JENKINS-50965] - Update Remoting to 3.21

* [JENKINS-51541] - Restrict SlaveComputer#setChannel(ChannelBuilder cb, …) to Beta-use only