Deprecate support for Gradle's maven plugin

Closes gh-25267

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/asciidoc/publishing.adoc

@@ -25,7 +25,7 @@ include::../gradle/publishing/maven-publish.gradle.kts[tags=publishing]
 
 [[publishing-your-application-maven]]
 === Publishing with the Maven Plugin
-WARNING: The `maven` plugin has been deprecated in Gradle 6 and has been removed in Gradle 7.
+WARNING: Due to its deprecation in Gradle 6, this plugin's support for publishing with Gradle's `maven` plugin is deprecated and will be removed in a future release.
 Please use the `maven-publish` plugin instead.
 
 When the {maven-plugin}[`maven` plugin] is applied, an `Upload` task for the `bootArchives` configuration named `uploadBootArchives` is automatically created.

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/asciidoc/reacting.adoc

@@ -66,4 +66,7 @@ When Gradle's {application-plugin}[`application` plugin] is applied to a project
 
 [[reacting-to-other-plugins-maven]]
 === Reacting to the Maven plugin
+WARNING: Support for reacting to Gradle's `maven` plugin is deprecated and will be removed in a future release.
+Please use the `maven-publish` plugin instead.
+
 When Gradle's {maven-plugin}[`maven` plugin] is applied to a project, the Spring Boot plugin will configure the `uploadBootArchives` `Upload` task to ensure that no dependencies are declared in the pom that it generates.

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/plugin/MavenPluginAction.java

@@ -1,5 +1,5 @@
 /*
- * 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.
@@ -19,14 +19,17 @@ package org.springframework.boot.gradle.plugin;
 import org.gradle.api.Action;
 import org.gradle.api.Plugin;
 import org.gradle.api.Project;
+import org.gradle.api.publish.maven.plugins.MavenPublishPlugin;
 import org.gradle.api.tasks.Upload;
 
 /**
  * {@link Action} that is executed in response to the
  * {@link org.gradle.api.plugins.MavenPlugin} being applied.
  *
+ * @deprecated since 2.5.0 in favor of using the {@link MavenPublishPlugin}
  * @author Andy Wilkinson
  */
+@Deprecated
 final class MavenPluginAction implements PluginApplicationAction {
 
 	private final String uploadTaskName;
@@ -36,7 +39,6 @@ final class MavenPluginAction implements PluginApplicationAction {
 	}
 
 	@Override
-	@SuppressWarnings("deprecation")
 	public Class<? extends Plugin<? extends Project>> getPluginClass() {
 		return org.gradle.api.plugins.MavenPlugin.class;
 	}
@@ -50,7 +52,6 @@ final class MavenPluginAction implements PluginApplicationAction {
 		});
 	}
 
-	@SuppressWarnings("deprecation")
 	private void clearConfigurationMappings(Upload upload) {
 		upload.getRepositories().withType(org.gradle.api.artifacts.maven.MavenResolver.class,
 				(resolver) -> resolver.getPom().getScopeMappings().getMappings().clear());

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/plugin/SpringBootPlugin.java

@@ -1,5 +1,5 @@
 /*
- * 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.
@@ -116,6 +116,7 @@ public class SpringBootPlugin implements Plugin<Project> {
 
 	private void registerPluginActions(Project project, Configuration bootArchives) {
 		SinglePublishedArtifact singlePublishedArtifact = new SinglePublishedArtifact(bootArchives.getArtifacts());
+		@SuppressWarnings("deprecation")
 		List<PluginApplicationAction> actions = Arrays.asList(new JavaPluginAction(singlePublishedArtifact),
 				new WarPluginAction(singlePublishedArtifact), new MavenPluginAction(bootArchives.getUploadTaskName()),
 				new DependencyManagementPluginAction(), new ApplicationPluginAction(), new KotlinPluginAction());