Pin Antora to 3.1.10 to fix deploy regression (#4155)

Antora 3.1.15 (published May 26) breaks remote content fetching,
causing the deploy workflow to produce zero output. Pin to the
known-working 3.1.10 release.

Author: kemister85

package.json

@@ -34,8 +34,8 @@
     "liquidjs": ">=10.25.7"
   },
   "devDependencies": {
-    "@antora/cli": "^3.1.10",
-    "@antora/site-generator-default": "^3.1.10",
+    "@antora/cli": "3.1.10",
+    "@antora/site-generator-default": "3.1.10",
     "@tinymce/antora-extension-livedemos": "^0.1.0",
     "@tinymce/moxiedoc": "^0.3.0",
     "dom-to-semantic-markdown": "^1.5.0",