repo: milestone: make /milestone/:id endpoint accessible (#11264) (#11282)
Previously, this required authentication, but there's not actually
any privileged information on this page. Move the endpoint out of
the group that requires sign-in. It still requires the ability to
read issues and pull requests, so private repositories (for instance)
will not be exposed.
Fixes #10312
Fixes #11233
Co-authored-by: guillep2k <18600385+guillep2k@users.noreply.github.com>