Revert "Merge branch 'confdential_epics_removal' into 'master'"

doc/api/epics.md

doc/api/graphql/reference/gitlab_schema.graphql

doc/api/graphql/reference/gitlab_schema.json

doc/user/group/epics/manage_epics.md

ee/app/controllers/groups/epics_controller.rb

ee/app/controllers/groups/roadmap_controller.rb

...hql/mutations/concerns/mutations/shared_epic_arguments.rb

ee/app/serializers/epic_entity.rb

ee/app/services/epics/create_service.rb

ee/app/services/epics/update_service.rb

ee/lib/api/epics.rb

ee/spec/controllers/groups/epics_controller_spec.rb

ee/spec/helpers/ee/issuables_helper_spec.rb

ee/spec/requests/api/epics_spec.rb

ee/spec/requests/api/graphql/mutations/epics/create_spec.rb

ee/spec/requests/api/graphql/mutations/epics/update_spec.rb

ee/spec/services/epics/create_service_spec.rb

ee/spec/services/epics/update_service_spec.rb