Resolve "Incorrect header validation in audit event streams"

What does this MR do and why?

Removing valid audit streaming header might make previously invalid row (which had the same name) valid

This MR adds handling of such scenario

Screenshots or screen recordings

Before:

Audit_events___Commit451___GitLab__1_

• Deleting row "removes" last letter
• After save the header is gone (this is because deleted and added header name clashes

After:

Audit_events___Commit451___GitLab__2_

• Removing row correctly removes invalid marker
• Header does not disappear

How to set up and validate locally

• Open Secure -> Audit events
• Create streaming destination
• Manipulate headers

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #418197 (closed)