PLT-5943 Reduce minimum username length (#5798)

model/user.go

@@ -36,7 +36,7 @@ const (
 	USER_LAST_NAME_MAX_RUNES  = 64
 	USER_AUTH_DATA_MAX_LENGTH = 128
 	USER_NAME_MAX_LENGTH      = 64
-	USER_NAME_MIN_LENGTH      = 3
+	USER_NAME_MIN_LENGTH      = 1
 )
 
 type User struct {