Harmonize settings.gradle documentation in Gradle getting started guide

See gh-17736

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

 pluginManagement {
 	repositories {
-		maven {
-			url 'https://repo.spring.io/libs-milestone'
-		}
+		maven { url 'https://repo.spring.io/milestone' }
+		gradlePluginPortal()
 	}
 	resolutionStrategy {
 		eachPlugin {

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

 pluginManagement {
 	repositories {
-		maven {
-			url = uri("https://repo.spring.io/libs-milestone")
-		}
+		maven { url = uri("https://repo.spring.io/milestone") }
+		gradlePluginPortal()
 	}
 	resolutionStrategy {
 		eachPlugin {

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

@@ -7,7 +7,7 @@ pluginManagement {
 	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}"
 			}
 		}
 	}

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

 pluginManagement {
 	repositories {
-		maven {
-			url = uri("https://repo.spring.io/libs-snapshot")
-		}
+		maven { url = uri("https://repo.spring.io/milestone") }
+		maven { url = uri("https://repo.spring.io/snapshot") }
+		gradlePluginPortal()
 	}
 	resolutionStrategy {
 		eachPlugin {