Spring Replace deprecated references in WebSocketMessageBrokerStats.toString()

This PR replaces deprecated references in the WebSocketMessageBrokerStats.toString().

• See gh-31604

Comment From: izeye

@rstoyanchev No, I just wanted to clean up the deprecated references.

Comment From: rstoyanchev

Okay, thanks for the suggestion. We can defer that until the deprecations are removed since it is internal only.