Update samples to use @SpringBootApplication

Update the relevant samples to use the new @SpringBootApplication
annotation.

Closes gh-1842

spring-boot-samples/spring-boot-sample-jta-jndi/src/main/java/sample/jndi/SampleJndiApplication.java

@@ -16,13 +16,9 @@
 
 package sample.jndi;
 
-import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
-import org.springframework.context.annotation.ComponentScan;
-import org.springframework.context.annotation.Configuration;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
 
-@Configuration
-@EnableAutoConfiguration
-@ComponentScan
+@SpringBootApplication
 public class SampleJndiApplication {
 
 }