Create extension to configure Spring maven repositories

Create a small Groovy script that can be used in `settings.gradle`
files to extend `repositories` to support the various maven
repositories required for our build.

See gh-42333

buildSrc/settings.gradle

@@ -1,14 +1,15 @@
 pluginManagement {
+	new File(rootDir.parentFile, "gradle.properties").withInputStream {
+		def properties = new Properties()
+		properties.load(it)
+		properties.forEach(settings.ext::set)
+		gradle.rootProject {
+			properties.forEach(project.ext::set)
+		}
+	}
+	evaluate(new File("${rootDir}/SpringRepositorySupport.groovy")).apply(this)
 	repositories {
 		mavenCentral()
 		gradlePluginPortal()
 	}
 }
-
-gradle.rootProject((project) -> {
-	new File(rootDir.parentFile, "gradle.properties").withInputStream {
-		def properties = new Properties()
-		properties.load(it)
-		properties.forEach(project.ext::set)
-	}
-});