reverting bump to 2021 in utilities package supported unity version

Author: LPLafontaineB

Packages/com.unity.multiplayer.samples.coop/package.json

@@ -4,7 +4,7 @@
   "version": "1.3.0-pre",
   "type": "template",
   "host": "hub",
-  "unity": "2021.3",
+  "unity": "2020.3",
   "description": "Utilities package built on top of Netcode for GameObjects, providing useful scripts and tools.",
   "dependencies": {
     "com.unity.learn.iet-framework": "1.2.1",