Do not enable H2C by default when using Reactor Netty

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

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/netty/NettyReactiveWebServerFactory.java

@@ -161,13 +161,8 @@ public class NettyReactiveWebServerFactory extends AbstractReactiveWebServerFact
 	}
 
 	private HttpProtocol[] listProtocols() {
-		if (getHttp2() != null && getHttp2().isEnabled()) {
-			if (getSsl() != null && getSsl().isEnabled()) {
-				return new HttpProtocol[] { HttpProtocol.H2, HttpProtocol.HTTP11 };
-			}
-			else {
-				return new HttpProtocol[] { HttpProtocol.H2C, HttpProtocol.HTTP11 };
-			}
+		if (getHttp2() != null && getHttp2().isEnabled() && getSsl() != null && getSsl().isEnabled()) {
+			return new HttpProtocol[] { HttpProtocol.H2, HttpProtocol.HTTP11 };
 		}
 		return new HttpProtocol[] { HttpProtocol.HTTP11 };
 	}