doc: Explain that node version 18 is needed, not version 20 (openscd#1663)

Co-authored-by: Christopher Lepski <[email protected]>

Author: Tamriel

CONTRIBUTING.md

@@ -37,7 +37,7 @@ To develop, follow these steps :
 1. Install [↗ Node.js](https://nodejs.org/en/download/package-manager)
 
 > [!IMPORTANT]  
-> `Node.js` version should be set to `20.x.x` as there are incompatibilities with higher version
+> `Node.js` version should be set to `18.x.x` as there are incompatibilities with higher version
 
 2. Run `npm ci` in OpenSCD's root folder.
 
@@ -355,4 +355,4 @@ class MyClass {
   private foo = 1;
   private bar() {}
 }
-```
+```