Add JUnit 4 and the Vintage Engine sample

This commit adds a sample to verify the use of JUnit 4 and the Vintage
Engine.

See gh-17100

spring-boot-samples/pom.xml

@@ -57,6 +57,7 @@
 		<module>spring-boot-sample-jta-bitronix</module>
 		<module>spring-boot-sample-jta-jndi</module>
 		<module>spring-boot-sample-junit-jupiter</module>
+		<module>spring-boot-sample-junit-vintage</module>
 		<module>spring-boot-sample-kafka</module>
 		<module>spring-boot-sample-liquibase</module>
 		<module>spring-boot-sample-logback</module>