Это действительно для более старой версии, например.1.5:
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>jaxb2-maven-plugin</artifactId>
<version>1.5</version>
<executions>
<execution>
<id>xjc</id>
<goals>
<goal>xjc</goal>
</goals>
<configuration>
<schemaDirectory>${project.basedir}src/main/resources</schemaDirectory>
<outputDirectory>${project.basedir}src/main/java</outputDirectory>
</configuration>
</execution>
</executions>
</plugin>
Для версии 2.4, согласно документации, больше нет тега schemaDirectory
https://www.mojohaus.org/jaxb2-maven-plugin/Documentation/v2.4/xjc-mojo.html
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>jaxb2-maven-plugin</artifactId>
<version>2.4</version>
<executions>
<execution>
<id>xjc</id>
<goals>
<goal>xjc</goal>
</goals>
</execution>
</executions>
<configuration>
<outputDirectory>${project.basedir}src/main/java</outputDirectory>
<sources>
<source>${project.basedir}src/main/resources</source>
</sources>
</configuration>
</plugin>