Start building against Spring Kafka 2.3.8 snapshots

See gh-20952

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

@@ -193,7 +193,7 @@
 		<spring-framework.version>5.2.6.BUILD-SNAPSHOT</spring-framework.version>
 		<spring-hateoas.version>1.0.4.RELEASE</spring-hateoas.version>
 		<spring-integration.version>5.2.6.BUILD-SNAPSHOT</spring-integration.version>
-		<spring-kafka.version>2.3.7.RELEASE</spring-kafka.version>
+		<spring-kafka.version>2.3.8.BUILD-SNAPSHOT</spring-kafka.version>
 		<spring-ldap.version>2.3.2.RELEASE</spring-ldap.version>
 		<spring-restdocs.version>2.0.4.RELEASE</spring-restdocs.version>
 		<spring-retry.version>1.2.5.RELEASE</spring-retry.version>