[Feature flag] Rollout audit event streaming custom headers UI

What does this MR do and why?

This MR removes the custom_headers_streaming_audit_events_ui feature flag from the codebase and reformats the documentation to unify the different parts and make it easier to reason with.

Screenshots or screen recordings

Blank	Listing	Add	Edit

How to set up and validate locally

1. Visit a group's audit events page e.g. http://gdk.test:3000/groups/gitlab-org/-/audit_events and look at the Streams tab.
2. Add a new streaming destination with and without headers and confirm both save successfully
3. Select the edit button on a destination item
4. Change the values of the headers for the destination item, add new headers, delete existing headers, and update existing headers.
5. Select the save button and confirm the changes were saved successfully.
6. Select the delete button on a destination item and confirm the destination is deleted successfully.
7. Using your browser's dev tools, block all requests to our /api/graphql endpoint and add a new destination URL with headers data. Check that the saving error message is shown and no headers were saved.
8. Unblock the /api/graphql endpoint.
9. Add a new destination URL with headers data but fill in the Value column for one of the headers with a space character. Check that the value validation error message is shown, no headers were saved and the destination was deleted (doesn't show up in the list).

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 #365259 (closed) #368515 (closed)