Perform validations on the parent of a namespace/group by default

This change validates the parent of a
namespace/group by default.

The validations are:

- A namesapce cannot have a parent
- A group can or cannot have a parent. If it has
a parent, it shoud be a group and not a namespace.

Changelog: added

app/models/namespace.rb

@@ -77,7 +77,7 @@ class Namespace < ApplicationRecord
 
   validates :max_artifacts_size, numericality: { only_integer: true, greater_than: 0, allow_nil: true }
 
-  validate :validate_parent_type, if: -> { Feature.enabled?(:validate_namespace_parent_type) }
+  validate :validate_parent_type, if: -> { Feature.enabled?(:validate_namespace_parent_type, default_enabled: :yaml) }
   validate :nesting_level_allowed
   validate :changing_shared_runners_enabled_is_allowed
   validate :changing_allow_descendants_override_disabled_shared_runners_is_allowed

config/feature_flags/development/validate_namespace_parent_type.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/322101
 milestone: '13.10'
 type: development
 group: group::access
-default_enabled: false
+default_enabled: true

doc/user/group/index.md

@@ -769,3 +769,22 @@ If a user sees a 404 when they would normally expect access, and the problem is
 
 In viewing the log entries, compare the `remote.ip` with the list of
 [allowed IPs](#restrict-group-access-by-ip-address) for the group.
+
+### Validation errors on namespaces and groups
+
+[GitLab 14.4 and later](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/70365) performs
+the following checks when creating or updating namespaces or groups:
+
+- Namespaces must not have parents.
+- Group parents must be groups and not namespaces.
+
+You can disable the validation if GitLab shows the following errors:
+
+- `A user namespace cannot have a parent`.
+- `A group cannot have a user namespace as its parent`.
+
+To disable the validation,
+[disable the `validate_namespace_parent_type` flag](../../administration/feature_flags.md).
+
+In the unlikely event that you had to disable this feature flag to prevent errors,
+[contact Support](https://about.gitlab.com/support/) so that we can improve this validation.