Increase token preview length in runner audit logs

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR adds logic to determine the correct length for a safe preview of the runner registration token that is shown in an audit log message, given that there was a recent introduction of a runner registration prefix.

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How to set up and validate locally

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1. Register a runner
2. Go to http://gdk.test:3000/admin/audit_logs
3. The runner registration even should contain a longer version of the registration token used, instead of just the first 9 characters, if a prefix is being used.

MR acceptance checklist

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