Ensure audits for user add-on assignments on association deletion

What does this MR do and why?

Ensure audits for user add-on assignments on association deletion. As PaperTrail creates only audits, when user add-on assignments are deleted directly, we need to adapt the audit creation process for deletion via associations (user, add-on purchases, etc.)

From now on we audit every destroy of user add-on assignments. We still don't track, when an engineer is using delete or delete_all on user add-on assignments.

References

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• Resolves #514313 (closed)

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Create a group
2. Go to Settings -> Billing
3. Apply for an Ultimate + Duo Enterprise Trial
4. Assign a Duo Enterprise seat to the owner of the group
5. Check on the Rails console that there is a new GitlabSubscriptions::UserAddOnAssignmentVersion for the event create
6. Delete the group
7. Check on the Rails console that there is a new GitlabSubscriptions::UserAddOnAssignmentVersion for the event destroy