Add mavenCentral() to pluginManagement repositories

Update the pluginManagement repositories used by Grade to include
`mavenCentral()` since `gradlePluginPortal()` has been suffering
from timeouts recently.

settings.gradle

@@ -1,5 +1,6 @@
 pluginManagement {
 	repositories {
+		mavenCentral()
 		gradlePluginPortal()
 		maven { url "https://repo.spring.io/release" }
 	}