[security][security sig] Retiring the security-specs repo

Hey folks, The security SIG has been going through the current list of repositories that we are managing and one of them, the "security-specs"[0] repository, has been unused for several years now so the consensus was to retire the repo. If there are no objections, we will look to retire the repo here relatively soon. Should anyone have any other information about it, please let the security SIG know. Thanks! [0] https://github.com/openstack/security-specs