chore: update doclet version

Same as googleapis/java-shared-config#368

Author: eaball35

pom.xml

@@ -605,7 +605,7 @@
       </activation>
       <properties>
         <!-- default config values -->
-        <docletName>java-docfx-doclet-1.3.0</docletName>
+        <docletName>java-docfx-doclet-1.4.0</docletName>
         <outputpath>${project.build.directory}/docfx-yml</outputpath>
         <projectname>${project.artifactId}</projectname>
         <excludeclasses></excludeclasses>