Support disabled deletion section in group/project settings

Depends on Add the allow_immediate_namespaces_deletion app... (!205554 - merged).

What does this MR do and why?

This change replaces a feature flag system with a new application setting to control when users can immediately delete groups and projects that are scheduled for deletion.

Previously, there was a feature flag called disallow_immediate_deletion that could prevent users from bypassing the normal deletion delay period. Now, there's a new instance-wide setting called "allow immediate namespaces deletion" that administrators can configure.

The key improvements include:

• Administrators can now control this behavior through the admin settings interface instead of feature flags
  • Being added in !205556 (merged)
• The UI now shows clearer messages and links to docs when immediate deletion is not available
• The system now checks if parent groups are also scheduled for deletion, which can block immediate deletion of child items
• Administrators always retain the ability to immediately delete items regardless of the setting

The default behavior allows immediate deletion on self-managed instances, but the setting is disabled on GitLab.com and GitLab Dedicated. This gives organizations more control over their deletion policies while providing better user experience through clearer messaging about what actions are available and why certain restrictions exist.

The feature is behind by the allow_immediate_namespaces_deletion feature flag. Documentation has been updated to explain the new setting, and tests have been added to verify the functionality works correctly.

References

• Related to Support disabled state in group/project settings (#570696)
• Part of Introduce a new "Allow immediate deletion" appl... (#569453)

Screenshots or screen recordings

As an admin

State	Screenshot
Group
Group pending deletion
Subgroup with parent pending deletion
Project
Project pending deletion
Project with parent pending deletion

As an owner of a group/project

State	Screenshot
Group
Subgroup with parent pending deletion
Project
Project with parent pending deletion

When allow_immediate_namespaces_deletion feature flag is enabled and Immediate deletion setting is disabled

State	Screenshot
Group pending deletion
Project pending deletion

When allow_immediate_namespaces_deletion feature flag is enabled and Immediate deletion setting is enabled

State	Screenshot
Group pending deletion
Project pending deletion

When allow_immediate_namespaces_deletion feature flag is disabled

State	Screenshot
Group pending deletion
Project pending deletion

How to set up and validate locally

1. In /rails/features enable the allow_immediate_namespaces_deletion feature flag
2. Open Rails console - bin/rails console
3. Run ApplicationSetting.first.update!(allow_immediate_namespaces_deletion: false)
4. Create a group
5. Create a subgroup and project in that group
6. Add a user as an owner to the top level group
7. Go to /admin/users and search for the user you just added as an owner
8. Impersonate the user (button in top right corner)
9. Go to group -> Settings -> General -> Advanced
10. Delete the group
11. You should not be able to delete the group immediately
12. Go to the subgroup and project, you should not be able to delete either immediately
13. Restore the top level group
14. Go to the project -> Settings -> General -> advanced
15. Delete the project
16. You should not be able to delete the project immediately

MR acceptance checklist

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