Fix sorting issue in user retrieval function

Modified the 'GetUsers()' function call in 'topup.go' to use 'GetUsersSorted()' instead. The change is expected to return users in a sorted manner which will significantly improve the feature that relies on the user list. Prior to this, the absence of sorting resulted in inefficient retrieval and handling of user list. Detailed error handling for potential issues during retrieval is yet to be implemented and is planned for future updates.