Start building against Spring Integration 5.1.8 snapshots

See gh-18313

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

@@ -172,7 +172,7 @@
 		<spring-data-releasetrain.version>Lovelace-SR10</spring-data-releasetrain.version>
 		<spring-framework.version>${spring.version}</spring-framework.version>
 		<spring-hateoas.version>0.25.2.RELEASE</spring-hateoas.version>
-		<spring-integration.version>5.1.7.RELEASE</spring-integration.version>
+		<spring-integration.version>5.1.8.BUILD-SNAPSHOT</spring-integration.version>
 		<spring-kafka.version>2.2.8.RELEASE</spring-kafka.version>
 		<spring-ldap.version>2.3.2.RELEASE</spring-ldap.version>
 		<spring-plugin.version>1.2.0.RELEASE</spring-plugin.version>