Remove flag that hides event streaming from graphQL schema

What does this MR do and why?

• Removes the feature flag that hides the group -> externalAuditEventDestinations link from the GraphQL schema.
• Gating of this feature is still handled by a policy, but the field needs to exist in the public schema to enable a gradual rollout.
• This feature is still not considered released as it is behind a default-off flag, but I've added a changelog trailer since it does technically update the public-facing API schema.

How to set up and validate locally

• Ensure that the ff_external_audit_events_namespace feature flag is disabled.
• Ensure that the group -> exteralAuditEventDestinations link in the GraphQL schema exists, but returns null if the flag is disabled.

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.