Start building against Spring Framework 5.2.8 snapshots

See gh-22372

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

@@ -191,7 +191,7 @@
 		<spring-batch.version>4.2.4.RELEASE</spring-batch.version>
 		<spring-cloud-connectors.version>2.0.7.RELEASE</spring-cloud-connectors.version>
 		<spring-data-releasetrain.version>Moore-SR8</spring-data-releasetrain.version>
-		<spring-framework.version>5.2.7.RELEASE</spring-framework.version>
+		<spring-framework.version>5.2.8.BUILD-SNAPSHOT</spring-framework.version>
 		<spring-hateoas.version>1.0.5.RELEASE</spring-hateoas.version>
 		<spring-integration.version>5.2.7.RELEASE</spring-integration.version>
 		<spring-kafka.version>2.3.9.RELEASE</spring-kafka.version>