Use static known URL for ExternalAuditEventDestination factory

What does this MR do and why?

Previously, a random generated URL could point to a valid IP which might be considered as "local network" or "loopback" by Gitlab::UrlBlocker and fail during record creation with:

Validation failed: Destination url is blocked: Requests to loopback addresses are not allowed

example.org is a known, static and public URL.

Fixes #343032 (closed)

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