Refactor target_id on AuditEvent

What does this MR do?

• Adds a target_id column to audit_events
• Adds target_id to the list of parallel persisted columns in AuditEvent
• Refactors all known write paths to target_id to use an integer. (😅)
• Adds missing specs discovered during refactor

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Note to @mwoolf: This will need thorough e2e manual testing before initial backend review.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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
• Security reports checked/validated by a reviewer from the AppSec team

Mentions #220322 (closed)