Add JUnit Jupiter API to spring-boot-docs build

This is required for the docs module to inspect Spring Boot
annotations for meta-annotations.

See gh-13739

spring-boot-project/spring-boot-docs/pom.xml

@@ -318,6 +318,11 @@
 			<artifactId>junit</artifactId>
 			<optional>true</optional>
 		</dependency>
+		<dependency>
+			<groupId>org.junit.jupiter</groupId>
+			<artifactId>junit-jupiter-api</artifactId>
+			<optional>true</optional>
+		</dependency>
 		<dependency>
 			<groupId>net.sf.ehcache</groupId>
 			<artifactId>ehcache</artifactId>