Tell maven-javadoc-plugin to target JDK 8

This unbreaks `mvn install` on AdoptOpenJDK 11.
Presumably it unbreaks on all JDKs >= 11.

Fixes #429

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=288016119

core/pom.xml

@@ -83,6 +83,7 @@
       <plugin>
         <artifactId>maven-javadoc-plugin</artifactId>
         <configuration>
+          <source>8</source>
           <encoding>UTF-8</encoding>
           <docencoding>UTF-8</docencoding>
           <charset>UTF-8</charset>