InfluxDB 2 API: list Users 'limit' query parameter does not work

__Steps to reproduce:__
List the minimal actions needed to reproduce the behavior.

1. Create more than 20 users (I've done this across multiple organisations but not sure if that is relevent)
2. Execute GET /api/v2/users?limit=50
3. Only 20 users are returned

__Expected behavior:__
All users are returned up to the limit specified in the query parameter.

__Actual behavior:__
Only 20 users are returned. It appears the 'limit' parameter is ignored.

__Environment info:__

* System info: Linux 5.4.0-80-generic x86_64
* InfluxDB version: InfluxDB 2.0.6 (git: 4db98b4c9a) build_date: 2021-04-29T16:48:12Z
* Other relevant environment details: Running within a docker container: docker engine version 20.10.7

Start by tracing the GET /api/v2/users handler and how it reads the limit query parameter. Add regression coverage for a request with limit=50 and verify that the response returns up to the requested number of users rather than the default 20.

Written by the indexing model from the issue text.