Clarify deprecation for Red Hat Enterprise Linux 7

In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/169713+ we
have deprecated Omnibus builds for RHEL 7 compatible operating systems.

We have never explicitly listed RHEL 7 as a supported operating
system, but should include it in the deprecations to avoid confusion.

https://docs.gitlab.com/ee/administration/package_information/supported_os.html#red-hat-enterprise-linux