PLANNER-2637: Upgrade to Antora 3

Need to upgrade Node version since Javascript introduced
new syntax the old version could not recognized. Because
eirslett/frontend-maven-plugin#1005
was resolved, I was able to remove the exec plugin and replace
it with a npx execution in the frontend-maven-plugin. Changed
the ui-bundle to the Antora 3 ui-bundle. Updated supplemental-ui
to match new expected layout.

Because Lunr extension is officially supported by Antora, its license
is changed from MIT to MPL-2.0 (which matches the license the
supplemental-ui and the ui-bundle zip are under).

optaplanner-website-docs/antora-playbook-author.yml

@@ -16,4 +16,7 @@ content:
 ui:
   bundle:
     url: ./ui-bundle/ui-bundle.zip
-  supplemental_files: ./supplemental-ui
+  supplemental_files: ./supplemental-ui
+antora:
+  extensions:
+    - '@antora/lunr-extension'

optaplanner-website-docs/antora-playbook.yml

@@ -20,3 +20,6 @@ ui:
   supplemental_files: ./supplemental-ui
 runtime:
   fetch: true
+antora:
+  extensions:
+    - '@antora/lunr-extension'