Update the URL to the upper-constraints file to point to the redirect
rule on releases.openstack.org so that anyone working on this branch
will switch to the correct upper-constraints list automatically when
the requirements repository branches.

Until the requirements repository has as stable/2024.2 branch, tests will
continue to use the upper-constraints list on master.

Change-Id: I03e63ea4a6a3080c28baa162e91e97319ae2d872