Upgrade to Netty 4.1.38 and Checkstyle 8.23

Author: jhoeller

build.gradle

@@ -37,7 +37,7 @@ ext {
 	junit5Version        = "5.3.2"
 	kotlinVersion        = "1.2.71"
 	log4jVersion         = "2.11.2"
-	nettyVersion         = "4.1.37.Final"
+	nettyVersion         = "4.1.38.Final"
 	reactorVersion       = "Californium-SR10"
 	rxjavaVersion        = "1.3.8"
 	rxjavaAdapterVersion = "1.2.1"
@@ -143,7 +143,7 @@ configure(allprojects) { project ->
 	}
 
 	checkstyle {
-		toolVersion = "8.22"
+		toolVersion = "8.23"
 		configDir = rootProject.file("src/checkstyle")
 	}