Log Audit Events for removal of approvers
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MR: Pending
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## Description
When changes happen in group membership and make users inherited members, users who were in an MR approval group disappear. There may be other membership changes that cause a similar where the approvers list is emptied.
There is no audit event indicating that the users were removed, but we have audit events when they are added to an approval group. This makes debugging when and by whom they were removed hard to track.
If there's an event to update the MR approval group list when users are removed, we can log that event and which user was removed. The users usually come back and
**Current behavior:**
No project audit event when users are removed due to changes in Direct/Inherited membership role.
**Expected behavior:**
When the membership structure changes, I would expect an audit event as below:
`Removed User Jane Doe from approval group on Example rule`
**Steps to reproduce:**
* Add two users to a project
* Add an approval rule with the two users,
* Move these users to be inherited members of the parent group instead of direct members.
* Their membership changes, they are removed from the approval rule, but no audit event indicates that update, whether expected or not.
**Additional information:**
Customers affected by an "unknown" cases of disappearing MR approvers where logs would have helped
* [Ticket](https://gitlab.zendesk.com/agent/tickets/604104) [604104](https://gitlab.zendesk.com/agent/tickets/604104)
* [Ticket 604547](https://gitlab.zendesk.com/agent/tickets/604547)
As a \[user or stakeholder\], I want \[goal or objective\] so that \[reason or benefit\].
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