Use app…DefaultJvmArgs as convention for start script's defaultJvmOpts

Closes gh-12631

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/plugin/ApplicationPluginActionIntegrationTests.gradle

@@ -8,6 +8,7 @@ apply plugin: 'org.springframework.boot'
 
 if (project.hasProperty('applyApplicationPlugin')) {
 	apply plugin: 'application'
+	applicationDefaultJvmArgs = ['-Dcom.example.a=alpha', '-Dcom.example.b=bravo']
 }
 
 task('taskExists') {
@@ -31,3 +32,11 @@ task('javaCompileEncoding') {
 		}
 	}
 }
+
+task('startScriptsDefaultJvmOpts') {
+	doFirst {
+		tasks.withType(org.springframework.boot.gradle.tasks.application.CreateBootStartScripts) {
+			println "$name defaultJvmOpts = $defaultJvmOpts"
+		}
+	}
+}