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

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content