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

@@ -9,8 +9,12 @@ allprojects {
 
 	repositories {
 		mavenCentral()
-		maven { url "https://repo.spring.io/milestone" }
+		if (!version.endsWith('RELEASE')) {
-		maven { url "https://repo.spring.io/snapshot" }
+			maven { url "https://repo.spring.io/milestone" }
+		}
+		if (version.endsWith('BUILD-SNAPSHOT')) {
+			maven { url "https://repo.spring.io/snapshot" }
+		}
 	}
 
 	configurations.all {

settings.gradle

@@ -2,7 +2,9 @@ pluginManagement {
 	repositories {
 		mavenCentral()
 		gradlePluginPortal()
-		maven { url "https://repo.spring.io/snapshot" }
+		if (version.endsWith('BUILD-SNAPSHOT')) {
+			maven { url "https://repo.spring.io/snapshot" }
+		}
 	}
 	resolutionStrategy {
 		eachPlugin {