EE: Require all templates to use default stages
What does this MR do?
Improves our templates and ensures that all of them are includeable and do define a default set of stages if overwritten.
Resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/59992
CE: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26954
Does this MR meet the acceptance criteria?
Conformity
Edited by Kamil Trzciński