Add runner unregistration audit event

Audit need

When admins, group/project owners/maintainers unregister a runner, it is important to preserve a trail of data of the event.

There are 2 ways a runner can be unregistered:

• Through the REST API, where we have an authentication_token which allows us to locate the runner, but not to know the identity of the caller
  • Direct destruction
  • Indirect destruction (e.g. by way of deleting a ci_runner_projects record / cascading delete)
• Through the GraphQL API/UI, where we know the identity of the PAT used for the API call

Proposal

Add the following audit events for when a GitLab Runner is unregistered at the instance, group, or project level.

• Date and timestamp in UTC
• Public IP address (i.e. the source IP address from where the registration command was run)
• Runner short SHA
• Author if available, otherwise the short authentication token