refactor: set javadoc-plugin to source version 1.8

pom.xml

@@ -91,6 +91,10 @@
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-javadoc-plugin</artifactId>
+                <configuration>
+                    <source>1.8</source>
+                    <target>1.8</target>
+                </configuration>
                 <executions>
                     <execution>
                         <id>attach-javadocs</id>