Stop building RHEL 7 and RHEL 7 compatible packages

Support for RHEL 7 and RHEL 7 compatible OSes has been deprecated, and was announced to be removed in %17.8. We should drop the CI jobs, and update the install page.

Note: We build RHEL 7 packages on a CentOS 7 builder. The package is published to RHEL 7, CentOS 7, Oracle Linux 7, and Scientific Linux repos.

Deliverables

• Drop the CI jobs that build and release CentOS 7 packages. !8097
• Drop RHEL 7, CentOS 7, Oracle Linux 7, and Scientific Linux reference from install page. gitlab-com/marketing/digital-experience/buyer-experience#4333
• Drop the CentOS 7 builder. gitlab-omnibus-builder!357
• Move the affected OSes to deprecated OSes: https://docs.gitlab.com/ee/administration/package_information/supported_os.html. gitlab!177094