Create audit event when the "Collect usage data" setting is changed

Proposal

The "Collect usage data" setting added in #587976 (closed) determines whether to share data important to the customer (including but not only full prompts and responses) collected from the customer's instance with GitLab.

However, no audit events are logged when this setting is changed. So it is impossible to track who changed the setting and when. This issue suggests logging an audit event whenever this setting is changed.