Enforce authentication via SSH certificates for Git access

Overview

More information about the motivation in epic: &11192 (closed)

Proposal

It should be possible to enforce authentication via SSH certificates for Git access by forbidding all other ways of authentication (user SSH key, access token, etc...). It can be something similar to https://docs.gitlab.com/ee/user/group/access_and_permissions.html#restrict-git-access-protocols or a completely new setting.