Fixup abstract configuration following SPR-16839

Spring Framework now requires that all nested configuration classes
are contained within a `@Component` class (see SPR-16839). This means
that our abstract `JpaBaseConfiguration` class should have a
`@Configuration` annotation.

Closes gh-14480

Author: philwebb

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/orm/jpa/JpaBaseConfiguration.java

@@ -70,6 +70,7 @@
  * @author Kazuki Shimizu
  * @author Eddú Meléndez
  */
+@Configuration
 @EnableConfigurationProperties(JpaProperties.class)
 @Import(DataSourceInitializedPublisher.Registrar.class)
 public abstract class JpaBaseConfiguration implements BeanFactoryAware {