Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/423509

**Problem**

The logic to convert repository size limit is defined on controller
level and service level. It leads to a double convertion problem.
Controller converts megabytes to bytes and then service repeats the
process.

However, it's broken only for create group flow. Because the update
group doesn't use a service.

**Solution**

* Use `Groups::UpdateService` to update groups from Admin area
* Remove byte convertion logic from controller

Changelog: fixed
EE: true