In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/35300 we fixed
how to handle group root Pages with mixed case path, but the
fix broke how we generate #pages_url for project level with mixed case.

This updates Project#pages_url so that we downcase only when comparing
with #pages_group_url and retrun the original url path if the project
is not group root Pages project.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/226874.