update node compatibility in docs per latest changes (#80)

Author: jelhan

README.md

@@ -14,7 +14,7 @@ Compatibility
 * Ember Changeset and Ember Changeset Validations v4
 * Ember.js v3.28 or above
 * Ember CLI v3.28 or above
-* Node.js v12 or above
+* Node.js v20 or above
 
 
 Installation