Stop using deprecated Gradle APIs

Closes gh-380

Author: wilkinsona

spring-javaformat-gradle/spring-javaformat-gradle-plugin/src/main/java/io/spring/javaformat/gradle/SpringJavaFormatPlugin.java

@@ -22,7 +22,7 @@
 import org.gradle.api.Project;
 import org.gradle.api.Task;
 import org.gradle.api.plugins.JavaBasePlugin;
-import org.gradle.api.plugins.JavaPluginConvention;
+import org.gradle.api.plugins.JavaPluginExtension;
 import org.gradle.api.tasks.SourceSet;
 import org.gradle.api.tasks.TaskContainer;
 import org.gradle.api.tasks.TaskProvider;
@@ -56,8 +56,8 @@ private void addSourceTasks() {
 			TaskProvider<Task> checkAllProvider = tasks.register(CheckFormat.NAME);
 			checkAllProvider.configure((checkAll) -> checkAll.setDescription(CheckFormat.DESCRIPTION));
 			tasks.named(JavaBasePlugin.CHECK_TASK_NAME).configure((check) -> check.dependsOn(checkAllProvider));
-			this.project.getConvention()
-				.getPlugin(JavaPluginConvention.class)
+			this.project.getExtensions()
+				.getByType(JavaPluginExtension.class)
 				.getSourceSets()
 				.all((sourceSet) -> addSourceTasks(sourceSet, checkAllProvider, formatAllProvider));
 		});