Suppress Kotlin 1.5.20 deprecation warning

See gh-27086

Author: philwebb

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

@@ -35,12 +35,17 @@ class KotlinPluginAction implements PluginApplicationAction {
 	public void execute(Project project) {
 		ExtraPropertiesExtension extraProperties = project.getExtensions().getExtraProperties();
 		if (!extraProperties.has("kotlin.version")) {
-			String kotlinVersion = project.getPlugins().getPlugin(KotlinPluginWrapper.class).getKotlinPluginVersion();
+			String kotlinVersion = getKotlinVersion(project);
 			extraProperties.set("kotlin.version", kotlinVersion);
 		}
 		enableJavaParametersOption(project);
 	}
 
+	@SuppressWarnings("deprecation")
+	private String getKotlinVersion(Project project) {
+		return project.getPlugins().getPlugin(KotlinPluginWrapper.class).getKotlinPluginVersion();
+	}
+
 	private void enableJavaParametersOption(Project project) {
 		project.getTasks().withType(KotlinCompile.class,
 				(compile) -> compile.getKotlinOptions().setJavaParameters(true));