Create a gerrit group to handle branches in Unmaintained status
across all projects, as described in TC resolution 2023-11-14,
which is commit 90982cd in the governance repository.
Also adjust the acl file normalization tool so that it will guarantee
that the Release Managers group has 'abandon' permission on
Unmaintained branches if any project chooses to override the global
openstack-unmaintained-core group with a project-specific unmaintained
core team (as is allowed by TC resolution 2023-11-14). This entails
a change in that script to require the acl file's namespace be passed
in so that the check doesn't affect non-OpenStack OpenInfra projects.
Change-Id: Ife8e5f175cb8a7d396dfe2a5d52fd6d524ae0b43
60de765fa0