Previously, Reactor Netty was the only embedded server that enabled
H2C by default. This commit updates the factory to only enable HTTP/2
when SSL has also been configured, aligning it with Jetty, Tomcat,
and Undertow.

If H2C is required, it can be enabled using a NettyServerCustomizer:

@Bean
NettyServerCustomizer h2cCustomizer() {
    return (httpServer) ->
           httpServer.protocol(HttpProtocol.HTTP11, HttpProtocol.H2C);
}

Closes gh-17867