Remove support for `AUTO_DEVOPS_DOMAIN`

KUBE_INGRESS_BASE_DOMAIN was introduced on https://gitlab.com/gitlab-org/gitlab-ce/issues/52363 / https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24580 deprecating AUTO_DEVOPS_DOMAIN. In the next major milestone (%12.0 ?), we need to remove support for AUTO_DEVOPS_DOMAIN from our codebase.

We need to also remove the ProjectAutoDevops#domain

Follow-up

• https://gitlab.com/gitlab-org/gitlab-ce/issues/62128

Workflow

If users have their own .gitlab-ci.yml, then they are un-affected.

If users use Auto DevOps, then those who have switched to setting the domain in cluster settings will be un-affected. (available since 11.8).

(Also note 11.8 will have migrated the data via a post-deploy migration)

For those who haven't done so, the Auto DevOps pipeline will fail with the following message:

In order to deploy or use Review Apps,
KUBE_INGRESS_BASE_DOMAIN variables must be set
From 11.8, you can set KUBE_INGRESS_BASE_DOMAIN in cluster settings
or by defining a variable at group or project level.
You can also manually add it in .gitlab-ci.yml