Revert "External Version bump for nixos new version string """

This reverts commit 68ca998e775c8c587038a8f34a2cdf7da6b4dc39.

roles/netbootxyz/defaults/main.yml

@@ -322,8 +322,22 @@ releases:
     menu: linux
     name: NixOS
     versions:
-    - code_name: null
-      name: null
+    - code_name: nixos-20.09
+      name: nixos-20.09
+    - code_name: nixos-21.05
+      name: nixos-21.05
+    - code_name: nixos-21.11
+      name: nixos-21.11
+    - code_name: nixos-22.05
+      name: nixos-22.05
+    - code_name: nixos-22.11
+      name: nixos-22.11
+    - code_name: nixos-23.05
+      name: nixos-23.05
+    - code_name: nixos-23.11
+      name: nixos-23.11
+    - code_name: nixos-unstable
+      name: nixos-unstable
   openEuler:
     base_dir: null
     enabled: true