Upgrade to Reactor Netty 1.2.8 for Spring Cloud Gateway

[lsc4719]

Upgrade to Reactor Netty 1.2.8 for Spring Cloud Gateway.

Now they support a new feature, a more appropriate way to implement HTTP/2:

Remove trailer header pre-declaration requirement by @violetagg in reactor/reactor-netty#3825

I'm using Spring Cloud Gateway to implement a proxy server that supports HTTP/2 protocol.
Reactor-Netty didn't support HTTP/2 trailer processing for proxy servers using Spring Cloud Gateway. 😿
So, Reactor-Netty corrected it. 😸

Spring Cloud Gateway said Spring Boot is the place to ask reactor-netty version upgrade.

[bclozel]

Duplicates #46275