Spring Use WebsocketServerSpec in ReactorNettyRequestUpgradeStrategy

ReactorNettyRequestUpgradeStrategy has properties that are passed as parameters to HttpServerResponse#sendWebSocket during WebSocket upgrades. In 0.9.5 Reactor Netty added WebsocketServerSpec as a way of passing any number of parameters. It is no longer possible to avoid use of this new API because it's the only way to pass some parameters, e.g. compress, based on #24903.

We can switch to WebsocketServerSpec and deprecate the properties in ReactorNettyRequestUpgradeStrategy which does mean that use of Reactor Netty 0.9.5 will be required as of 5.2.6 but that is highly advisable in any case due to CVE-2020-5403 and CVE-2020-5404.