Upgrade to Kotlin 1.2.51

Closes gh-13705

spring-boot-project/spring-boot-dependencies/pom.xml

@@ -115,7 +115,7 @@
 		<junit-jupiter.version>5.1.1</junit-jupiter.version>
 		<junit-platform.version>1.1.0</junit-platform.version>
 		<kafka.version>1.0.1</kafka.version>
-		<kotlin.version>1.2.41</kotlin.version>
+		<kotlin.version>1.2.51</kotlin.version>
 		<lettuce.version>5.0.4.RELEASE</lettuce.version>
 		<liquibase.version>3.5.5</liquibase.version>
 		<log4j2.version>2.10.0</log4j2.version>