Fix: relax Node engine requirement from >=22.22.0 to >=18.0.0 (#930)

The engine requirement was set to an unreleased Node version (22.22.0).
CI already tests and passes on Node 20. Etherpad supports Node >=18.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: JohnMcLear

package.json

@@ -140,6 +140,6 @@
     }
   ],
   "engines": {
-    "node": ">=22.22.0"
+    "node": ">=18.0.0"
   }
 }