fix javadoc plugin config

Crim · Crim · commit ba98ff375e32 · 2020-07-20T17:51:13.000+09:00
diff --git a/pom.xml b/pom.xml
@@ -248,6 +248,7 @@
                                 </goals>
                                 <configuration>
                                     <failOnWarnings>false</failOnWarnings>
+                                    <source>8</source>
                                 </configuration>
                             </execution>
                         </executions>
