Update conditional inclusion of milestone repository

The new Spring version schema does not include a "RELEASE" version.
This commit modifies the Gradle conditional to include the milestone
repository for all milestone and snapshot versions.

See gh-22506

build.gradle

@@ -17,7 +17,7 @@ allprojects {
 
 	repositories {
 		mavenCentral()
-		if (!version.endsWith('RELEASE')) {
+		if (version.contains('-')) {
 			maven { url "https://repo.spring.io/milestone" }
 		}
 		if (version.endsWith('-SNAPSHOT')) {