fix: Update doclet to latest version (#499)

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Fixes #<issue_number_goes_here> ☕️

If you write sample code, please follow the [samples format](
https://github.com/GoogleCloudPlatform/java-docs-samples/blob/main/SAMPLE_FORMAT.md).

pom.xml

@@ -694,7 +694,7 @@
       </activation>
       <properties>
         <!-- default config values -->
-        <docletName>java-docfx-doclet-1.5.0</docletName>
+        <docletName>java-docfx-doclet-1.6.0</docletName>
         <docletPath>${env.KOKORO_GFILE_DIR}/${docletName}.jar</docletPath>
         <outputpath>${project.build.directory}/docfx-yml</outputpath>
         <projectname>${project.artifactId}</projectname>