Upgrade to Tomcat 9.0.21, Undertow 2.0.21, RxJava 2.2.9, Checkstyle 8.21

Includes upgrade to Reactor Californium SR9 proper.

build.gradle

@@ -38,14 +38,14 @@ ext {
 	kotlinVersion        = "1.2.71"
 	log4jVersion         = "2.11.2"
 	nettyVersion         = "4.1.36.Final"
-	reactorVersion       = "Californium-BUILD-SNAPSHOT"
+	reactorVersion       = "Californium-SR9"
 	rxjavaVersion        = "1.3.8"
 	rxjavaAdapterVersion = "1.2.1"
-	rxjava2Version       = "2.2.8"
+	rxjava2Version       = "2.2.9"
 	slf4jVersion         = "1.7.26"	  // spring-jcl + consistent 3rd party deps
 	tiles3Version        = "3.0.8"
-	tomcatVersion        = "9.0.19"
-	undertowVersion      = "2.0.20.Final"
+	tomcatVersion        = "9.0.21"
+	undertowVersion      = "2.0.21.Final"
 
 	gradleScriptDir = "${rootProject.projectDir}/gradle"
 	withoutJclOverSlf4J = {
@@ -143,13 +143,12 @@ configure(allprojects) { project ->
 	}
 
 	checkstyle {
-		toolVersion = "8.20"
+		toolVersion = "8.21"
 		configDir = rootProject.file("src/checkstyle")
 	}
 
 	repositories {
 		maven { url "https://repo.spring.io/libs-release" }
-		maven { url "https://repo.spring.io/snapshot" } // Reactor
 		mavenLocal()
 	}