Configure Spring Milestone repository for apiDiff task

Prior to this commit, the `apiDiff` custom Gradle task would only use
the configured repositories in the build to generated the API diff
report. This causes issues when the report has to be generated against a
previous milestone: the Spring Framework build only relies on Maven
Central and a specific `libs-spring-framework-build` repository for
building the project. In this case, the task cannot resolve the previous
milestone artifacts to generate the report.

This commit improves the `ApiDiffPlugin` to automatically add the Spring
Milestone repository to the root project configuration when the task is
executed.

Fixes gh-27928

buildSrc/src/main/java/org/springframework/build/api/ApiDiffPlugin.java

@@ -16,6 +16,7 @@
 package org.springframework.build.api;
 
 import java.io.File;
+import java.net.URI;
 import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.util.Collections;
@@ -23,10 +24,14 @@ import java.util.List;
 
 import me.champeau.gradle.japicmp.JapicmpPlugin;
 import me.champeau.gradle.japicmp.JapicmpTask;
+import org.gradle.api.Action;
 import org.gradle.api.Plugin;
 import org.gradle.api.Project;
 import org.gradle.api.artifacts.Configuration;
 import org.gradle.api.artifacts.Dependency;
+import org.gradle.api.artifacts.repositories.ArtifactRepository;
+import org.gradle.api.artifacts.repositories.MavenArtifactRepository;
+import org.gradle.api.artifacts.repositories.RepositoryContentDescriptor;
 import org.gradle.api.plugins.JavaBasePlugin;
 import org.gradle.api.plugins.JavaPlugin;
 import org.gradle.api.publish.maven.plugins.MavenPublishPlugin;
@@ -52,6 +57,8 @@ public class ApiDiffPlugin implements Plugin<Project> {
 
 	private static final List<String> PACKAGE_INCLUDES = Collections.singletonList("org.springframework.*");
 
+	private static final URI SPRING_MILESTONE_REPOSITORY = URI.create("https://repo.spring.io/milestone");
+
 	@Override
 	public void apply(Project project) {
 		if (project.hasProperty(BASELINE_VERSION_PROPERTY) && project.equals(project.getRootProject())) {
@@ -68,6 +75,10 @@ public class ApiDiffPlugin implements Plugin<Project> {
 
 	private void createApiDiffTask(String baselineVersion, Project project) {
 		if (isProjectEligible(project)) {
+			// Add Spring Milestone repository for generating diffs against previous milestones
+			project.getRootProject()
+					.getRepositories()
+					.maven(mavenArtifactRepository -> mavenArtifactRepository.setUrl(SPRING_MILESTONE_REPOSITORY));
 			JapicmpTask apiDiff = project.getTasks().create(TASK_NAME, JapicmpTask.class);
 			apiDiff.setDescription("Generates an API diff report with japicmp");
 			apiDiff.setGroup(JavaBasePlugin.DOCUMENTATION_GROUP);