Set "proxy target class" for transaction managers

Update `@EnableTransactionManagement` so that `proxyTargetClass` is
set to true. This ensures that @Transactional beans that aren't
interface based can still be proxied.

Fixes gh-5423

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jdbc/DataSourceTransactionManagerAutoConfiguration.java

@@ -65,7 +65,7 @@ public class DataSourceTransactionManagerAutoConfiguration {
 
 	@ConditionalOnMissingBean(AbstractTransactionManagementConfiguration.class)
 	@Configuration
-	@EnableTransactionManagement
+	@EnableTransactionManagement(proxyTargetClass = true)
 	protected static class TransactionManagementConfiguration {
 
 	}