Make Server::setDefaultHandler and Server::setErrorHandler arguments consistent

[scrat98]

Jetty version(s)
12.0.16

Enhancement Description
There is an inconsistency in arguments between Server::setDefaultHandler and Server::setErrorHandler.

• Server::setDefaultHandler accepts org.eclipse.jetty.server.Handler
• Server::setErrorHandler accepts org.eclipse.jetty.server.Request.Handler

I would like to use in Application code only org.eclipse.jetty.server.Handler and if I do so and use it with Server::setErrorHandler then I'll get a warning
WARN o.e.jetty.server.Handler$Abstract - No Server set for csclhs.UnhandledErrorsHandler@5ace1ed4{STARTING} and due to this I need to implement my hanlder as a org.eclipse.jetty.server.Request.Handler to suppress this warning.

Nothing serious, but I think it should be easy to change.