Add auditEventsStreamingHeadersUpdate mutation

What does this MR do and why?

• Adds a GraphQL mutation to update a custom header for an event streaming destination.

Screenshots or screen recordings

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How to set up and validate locally

1. Enable the feature flag and create a header as described in this MR.
2. Instead of deleting the header, update it using the auditEventsStreamingHeadersUpdate mutation.
3. Assert that the header has updated successfully.
4. Assert that only a group owner is able to make this change.

MR acceptance checklist

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

Related to #361964 (closed)