Enable SNAPSHOT builds

To use Reactor SNAPSHOTs in gh-18041 we need to enable the
snapshot repositories.

Issue gh-18041

buildSrc/build.gradle

@@ -12,7 +12,7 @@ java {
 repositories {
 	gradlePluginPortal()
 	mavenCentral()
-	maven { url 'https://repo.spring.io/milestone' }
+	maven { url 'https://repo.spring.io/snapshot' }
 }
 
 sourceSets {

settings.gradle

@@ -11,7 +11,7 @@ plugins {
 dependencyResolutionManagement {
 	repositories {
 		mavenCentral()
-		maven { url "https://repo.spring.io/milestone" }
+		maven { url "https://repo.spring.io/snapshot" }
 	}
 }