Add missing repositories to snapshot configuration

See gh-17736

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/gradle/getting-started/snapshot-settings.gradle

@@ -1,13 +1,13 @@
 pluginManagement {
         repositories {
-		maven {
+                maven { url 'https://repo.spring.io/milestone' }
-			url 'https://repo.spring.io/libs-snapshot'
+                maven { url 'https://repo.spring.io/snapshot' }
-		}
+		gradlePluginPortal()
 	}
 	resolutionStrategy {
 		eachPlugin {
 			if (requested.id.id == 'org.springframework.boot') {
-				useModule "org.springframework.boot:spring-boot-gradle-plugin:${requested.version}"
+				useModule("org.springframework.boot:spring-boot-gradle-plugin:${requested.version}")
 			}
 		}
 	}