Update to antora-extensions 1.12.0

package.json

@@ -4,7 +4,7 @@
     "@antora/atlas-extension": "1.0.0-alpha.2",
     "@antora/collector-extension": "1.0.0-alpha.4",
     "@asciidoctor/tabs": "1.0.0-beta.6",
-    "@springio/antora-extensions": "1.11.1",
+    "@springio/antora-extensions": "1.12.0",
     "@springio/asciidoctor-extensions": "1.0.0-alpha.11"
   }
 }