Upgrade to Spring AMQP 2.1.8

Closes gh-17641

Author: snicoll

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

@@ -163,7 +163,7 @@
 		<solr.version>7.7.2</solr.version>
 		<!-- deprecated in favor of "spring-framework.version" -->
 		<spring.version>5.1.9.RELEASE</spring.version>
-		<spring-amqp.version>2.1.8.BUILD-SNAPSHOT</spring-amqp.version>
+		<spring-amqp.version>2.1.8.RELEASE</spring-amqp.version>
 		<spring-batch.version>4.1.2.RELEASE</spring-batch.version>
 		<spring-cloud-connectors.version>2.0.6.RELEASE</spring-cloud-connectors.version>
 		<spring-data-releasetrain.version>Lovelace-SR10</spring-data-releasetrain.version>