Remove the cert-based integration

The primary goal of this issue is to collect the blocking issues before we want to remove the certificate-based integrations altogether.

The official deprecation notice is the primary source of truth for dates.

For Self-Managed users, the integration will be available behind a feature flag at least

• until the linked issues are fixed in some form and
• three months have passed to allow migrating over to the GitLab Agent.

For SaaS users, the certificate-based integration will be switched off starting around 2022. May 22. After this event, only the current users of the integration will be able to use it. We will work with these users to migrate them over to the agent-based integrations as soon as possible.

Note: As some certificate-based features were only sparingly used and could not serve large user bases, we do not aim at feature-parity before the removal. An example that falls under this category is "Deploy boards". We consider the problem worth solving but are looking into an improved solution as our users often criticized our previous solution. Thus we are willing to remove the certificate-based integrations without providing an alternative for some use cases. At the same time, having an alternative is on our roadmap.