Move Gradle plugin repos declaration to settings

See gh-23282

build.gradle

@@ -1,7 +1,4 @@
 buildscript {
-	repositories {
-		maven { url 'https://repo.spring.io/plugins-release' }
-	}
 	dependencies {
 		classpath 'org.asciidoctor:asciidoctorj-pdf:1.5.0-alpha.16'
 		classpath 'io.spring.asciidoctor:spring-asciidoctor-extensions:0.1.3.RELEASE'

settings.gradle

@@ -1,3 +1,10 @@
+pluginManagement {
+	repositories {
+		gradlePluginPortal()
+		maven { url 'https://repo.spring.io/plugins-release' }
+	}
+}
+
 include "spring-aop"
 include "spring-aspects"
 include "spring-beans"