Stop building Raspberry Pi OS Buster packages

The deprecation of PiOS buster packages for 17.8 was annoucned in #8734 (closed). The PiOS buster CI config and documentation can be updated/removed in %17.8.

Acceptance Criteria

• PiOS Buster CI jobs removed !8105 (merged)
• PiOS Buster moved to deprecated OSes: https://docs.gitlab.com/ee/administration/package_information/supported_os.html. gitlab!177094 (merged)
• PiOS Buster removed from install page. gitlab-com/marketing/digital-experience/buyer-experience#4333 (closed)
• PiOS Buster builder image removed. gitlab-omnibus-builder!369 (merged)