Update node requirements

CONTRIBUTING.md

@@ -69,7 +69,7 @@ You can launch a binder with the latest JupyterLab master to test something (thi
 
 ### Installing Node.js and jlpm
 
-Building JupyterLab from its GitHub source code requires Node.js. The development version requires Node.js version 12+.
+Building JupyterLab from its GitHub source code requires Node.js. The development version requires Node.js version 10+.
 
 If you use `conda`, you can get it with:
 

dev_mode/package.json

@@ -184,7 +184,7 @@
     "react-dom": "~16.9.0"
   },
   "engines": {
-    "node": ">=6.11.5"
+    "node": ">=10.0.0"
   },
   "jupyterlab": {
     "name": "JupyterLab",

jupyterlab/staging/package.json

@@ -184,7 +184,7 @@
     "react-dom": "~16.9.0"
   },
   "engines": {
-    "node": ">=6.11.5"
+    "node": ">=10.0.0"
   },
   "jupyterlab": {
     "name": "JupyterLab",