CI: Run HTML5 export workflow on 3.x branch only

.github/workflows/export_html5.yml

@@ -2,7 +2,7 @@ name: Export projects to HTML5 and deploy to GitHub Pages
 on:
   push:
     branches:
-      - master
+      - 3.x
 
 env:
   GODOT_VERSION: 3.3.3