Deprecate support for Scientific Linux 7

Summary

1. Add deprecation warning
2. Announce deprecation in next release post
3. Remove from supported operating systems page / Add to no longer supported list

Note: Scientific Linux 7 packages use the packages built for CentOS so no CI jobs need to be changed/removed.

https://gitlab.com/gitlab-org/gitlab/-/blob/f429858ec31468e63d80dae62c4052690ab9c3cf/doc/administration/package_information/supported_os.md#supported-operating-systems