Configure repositories based on the version that is being built

https://repo.spring.io/snapshot should only be available when
building a BUILD-SNAPSHOT. https://repo.spring.io/milestone should
only be available when not building a RELEASE (a BUILD-SNAPSHOT, Mn,
or RCn).

Closes gh-19700

build.gradle

@@ -9,9 +9,13 @@ allprojects {
 
 	repositories {
 		mavenCentral()
+		if (!version.endsWith('RELEASE')) {
 			maven { url "https://repo.spring.io/milestone" }
+		}
+		if (version.endsWith('BUILD-SNAPSHOT')) {
 			maven { url "https://repo.spring.io/snapshot" }
 		}
+	}
 
 	configurations.all {
 		resolutionStrategy.cacheChangingModulesFor 60, "minutes"

settings.gradle

@@ -2,8 +2,10 @@ pluginManagement {
 	repositories {
 		mavenCentral()
 		gradlePluginPortal()
+		if (version.endsWith('BUILD-SNAPSHOT')) {
 			maven { url "https://repo.spring.io/snapshot" }
 		}
+	}
 	resolutionStrategy {
 		eachPlugin {
 			if (requested.id.id == "org.jetbrains.kotlin.jvm") {