Upgrade to Reactor Californium-SR21

Includes Undertow 2.0.31 and Checkstyle 8.36.1 as well.

Closes gh-25731

Author: jhoeller

build.gradle

@@ -38,14 +38,14 @@ ext {
 	kotlinVersion        = "1.2.71"
 	log4jVersion         = "2.11.2"
 	nettyVersion         = "4.1.51.Final"
-	reactorVersion       = "Californium-BUILD-SNAPSHOT"
+	reactorVersion       = "Californium-SR21"
 	rxjavaVersion        = "1.3.8"
 	rxjavaAdapterVersion = "1.2.1"
 	rxjava2Version       = "2.2.19"
 	slf4jVersion         = "1.7.30"	  // spring-jcl + consistent 3rd party deps
 	tiles3Version        = "3.0.8"
 	tomcatVersion        = "9.0.37"
-	undertowVersion      = "2.0.30.Final"
+	undertowVersion      = "2.0.31.Final"
 
 	gradleScriptDir = "${rootProject.projectDir}/gradle"
 	withoutJclOverSlf4j = {
@@ -142,14 +142,13 @@ configure(allprojects) { project ->
 	}
 
 	checkstyle {
-		toolVersion = "8.36"
+		toolVersion = "8.36.1"
 		configDir = rootProject.file("src/checkstyle")
 	}
 
 	repositories {
 		mavenCentral()
 		maven { url "https://repo.spring.io/libs-release" }
-		maven { url "https://repo.spring.io/snapshot" }	// Reactor
 	}
 
 	dependencies {