Publish Gradle metadata

This commit enables the publishing of Gradle metadata with Spring
Framework artifacts. This feature will be enabled by default with Gradle
6.0, so Spring Framework will consistently push this information in the
5.2.x generation.

Fixes gh-23503

settings.gradle

@@ -5,6 +5,8 @@ pluginManagement {
 	}
 }
 
+enableFeaturePreview("GRADLE_METADATA")
+
 include "spring-aop"
 include "spring-aspects"
 include "spring-beans"