Avoid duplicate entries

Prior to this commit, the schema distribution archive contains
duplicate entries for the more recent XSD as it is reference both with
its version and as the "default" XSD (i.e. without a version).

This commit configures the task to apply a duplicatesStrategy so that
only the first entry remains.

Issue: SPR-12011

build.gradle

@@ -1171,7 +1171,7 @@ configure(rootProject) {
 		classifier = "schema"
 		description = "Builds -${classifier} archive containing all " +
 			"XSDs for deployment at http://springframework.org/schema."
-
+		duplicatesStrategy 'exclude'
 		subprojects.each { subproject ->
 			def Properties schemas = new Properties();