Improve parallel persistance for audit event author_name

What does this MR do?

On verification of #220318 (closed) we discovered that some audit events were not created with author_name populated correctly.

This approach saves the author_name as an AR callback (similar to the approach agreed in !36198 (merged)) making it much more unlikely that this could happen in future.

It also brings consistency to the approach we're going to take in the parallel persistence phase of refactoring AuditEvent.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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Mentions #220318 (closed)