Stop overriding thread name prefix for externally configured Executor in ChannelRegistration

[snicoll]

AbstractMessageBrokerConfiguration uses ChannelRegistration to setup the following:

• clientInboundChannel with a thread name prefix of clientInboundChannel-
• clientOutboundChannel with a thread name prefix of clientOutboundChannel-
• brokerChannel with a thread name prefix of brokerChannel-

It does so even if the underlying Executor has been externally provided. This is problematic, especially with virtual threads as the same instance is probably shared by all three channels.