Deprecate support for CentOS 7

CentOS 7 will reach EOL in June 30 2024 as per https://www.redhat.com/en/topics/linux/centos-linux-eol. This makes GitLab 17.8 the last GitLab version for which we can provide packages for that OS. We should announce deprecation of the OS in 16.11 so that users get up to 17.8 to switch to a newer version of CentOS

• Decide the timeline and announce it in %17.0
• Add CentOS 7 to the deprecated OS list in OmnibusHelper so that reconfigure logs show warnings
• Add CentOS 7 deprecation similar to Add deprecation for Ubuntu 18.04 (gitlab!141344 - merged)
• Update supported OS docs page, also include a note to the known limitation