Generate Javadoc with same source level as used for compilation

This works around a change in javadoc generation in JDK 11.0.2. See
https://bugs.openjdk.java.net/browse/JDK-8212233 for details.

Author: wilkinsona

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

@@ -677,6 +677,9 @@
 					<plugin>
 						<groupId>org.apache.maven.plugins</groupId>
 						<artifactId>maven-javadoc-plugin</artifactId>
+						<configuration>
+							<source>${java.version}</source>
+						</configuration>
 						<executions>
 							<execution>
 								<id>attach-javadocs</id>