Updating private_membership metadata

# Providing 'This is the canonical public repository' for URL

We are moving towards more structured metadata and (for now) this a
magic phrase that means a few things:

* There is no meaningful external upstream.
* Different vulnerability tracking and response process.

Bug: 379211393
Change-Id: Ibbad83be48822fc46ad327f04ff2590f78b3639b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6290774
Reviewed-by: Sergiy Belozorov <sergiyb@chromium.org>
Reviewed-by: Rachael Newitt <renewitt@google.com>
Commit-Queue: Jordan Brown <rop@google.com>
Owners-Override: Jordan Brown <rop@google.com>
Cr-Commit-Position: refs/heads/main@{#1423737}