Require a milestone when upgrading dependencies

Closes gh-25225

buildSrc/src/main/java/org/springframework/boot/build/bom/bomr/UpgradeBom.java

 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -33,6 +33,7 @@ import org.gradle.api.DefaultTask;
 import org.gradle.api.InvalidUserDataException;
 import org.gradle.api.Task;
 import org.gradle.api.internal.tasks.userinput.UserInputHandler;
+import org.gradle.api.tasks.Input;
 import org.gradle.api.tasks.TaskAction;
 import org.gradle.api.tasks.TaskExecutionException;
 import org.gradle.api.tasks.options.Option;
@@ -64,6 +65,11 @@ public class UpgradeBom extends DefaultTask {
 		this.milestone = milestone;
 	}
 
+	@Input
+	public String getMilestone() {
+		return this.milestone;
+	}
+
 	@TaskAction
 	void upgradeDependencies() {
 		GitHubRepository repository = createGitHub().getRepository(this.bom.getUpgrade().getGitHub().getOrganization(),