Upgrade @springio/antora-xref-extension to 1.0.0-alpha.4

Closes gh-43043

Author: philwebb

antora/package-lock.json

@@ -7,7 +7,7 @@
     "@antora/site-generator": "3.2.0-alpha.4",
     "@antora/atlas-extension": "1.0.0-alpha.2",
     "@springio/antora-extensions": "1.11.1",
-    "@springio/antora-xref-extension": "1.0.0-alpha.3",
+    "@springio/antora-xref-extension": "1.0.0-alpha.4",
     "@springio/antora-zip-contents-collector-extension": "1.0.0-alpha.8",
     "@asciidoctor/tabs": "1.0.0-beta.6",
     "@springio/asciidoctor-extensions": "1.0.0-alpha.11"