Stop building Debian 10 packages

Debian 10 reaches EOL today(June 30 2024) as per https://www.debian.org/releases/buster/

Tasks:

1. Add deprecation warning (gitlab!160323 (merged)) (!7852 (merged)) (!7854 (merged))
2. Announce deprecation in next release post (gitlab-com/www-gitlab-com!135763 (merged))
3. Remove debian 10 jobs from pipeline (!7990 (merged))
4. Remove from supported operating systems page / Add to no longer supported list (gitlab!169249 (merged))
5. Remove from install page: https://about.gitlab.com/install (gitlab-com/marketing/digital-experience/buyer-experience#4164 (closed))