add rspack constraint

Author: vdusek

website/package-lock.json

@@ -27,6 +27,7 @@
         "@docusaurus/faster": "^3.8.1",
         "@docusaurus/plugin-client-redirects": "^3.8.1",
         "@docusaurus/preset-classic": "^3.8.1",
+        "@rspack/core": "<1.6.0",
         "@signalwire/docusaurus-plugin-llms-txt": "^1.2.2",
         "clsx": "^2.0.0",
         "docusaurus-gtm-plugin": "^0.0.2",