Emit audit event for instance SSH certificate creation

What does this MR do and why?

Emits audit event on instance SSH certificate creation.

References

Contributes to #611324 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. In gdk/env.runit, set export GITLAB_SIMULATE_SAAS=0, then gdk restart rails-web.
2. Enable the flag in rails c: Feature.enable(:instance_ssh_certificates).
3. Sign in as an admin with admin mode enabled and open http://gdk.test:3000/admin/ssh_certificates.
4. Select Add certificate authority. The form renders below the card header.
5. Generate a CA key with ssh-keygen -t ed25519 -f /tmp/ca -N '', enter a title and the contents of /tmp/ca.pub, and submit.
6. AuditEvents::InstanceAuditEvent.last returns an audit event for the newly created certificate, with the title and fingerprint logged in custom_message.

MR acceptance checklist

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