Add Apache's maven staging repository

This is required to build against Spring for Apache
Kafka 2.6.0-SNAPSHOT which brings kafka-clients:2.6.0.

kafka-clients:2.6.0 is actually an RC version and is
not promoted to central yet. The staging repository
will be removed once kafka-clients:2.6.0 is promoted
to maven central.

build.gradle

@@ -37,6 +37,7 @@ allprojects {
 	group = 'org.springframework.batch'
 
 	repositories {
+		maven { url 'https://repository.apache.org/content/groups/staging/' } // TODO to remove when kafka-clients 2.6.0 is promoted to central
 		maven { url 'https://repo.spring.io/libs-snapshot' }
 		maven { url 'https://repo.spring.io/libs-milestone' }
 		maven { url 'https://repo.spring.io/plugins-release' }