deps: updata java-docfx-doclet to v1.8.0 (#540)

Author: lqiu96

pom.xml

@@ -694,7 +694,7 @@
       </activation>
       <properties>
         <!-- default config values -->
-        <docletName>java-docfx-doclet-1.6.0</docletName>
+        <docletName>java-docfx-doclet-1.8.0</docletName>
         <docletPath>${env.KOKORO_GFILE_DIR}/${docletName}.jar</docletPath>
         <outputpath>${project.build.directory}/docfx-yml</outputpath>
         <projectname>${project.artifactId}</projectname>