update package.json and docs to release v3.5.21

CHANGES.md

@@ -14,6 +14,11 @@ Upgrading Guide: https://docs.growi.org/en/admin-guide/upgrading/36x.html
 
 * Improvement: Drop unnecessary MongoDB collection indexes
 
+## 3.5.21
+
+* Fix: Inviting user doesn't work
+    * Introduced by 3.5.20
+
 ## 3.5.20
 
 * Improvement: Organize MongoDB collection indexes uniqueness

package.json

@@ -1,6 +1,6 @@
 {
   "name": "growi",
-  "version": "3.6.0-RC",
+  "version": "3.5.21-RC",
   "description": "Team collaboration software using markdown",
   "tags": [
     "wiki",