Merge pull request #17736 from dsyer

* pr/17736:
  Harmonize settings.gradle documentation in Gradle getting started guide
  Add missing repositories to snapshot configuration

Closes 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

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

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 {