Add group setting: allow_non_enterprise_placeholder_reassignment

What does this MR do and why?

Introduce allow_non_enterprise_placeholder_reassignment as an Owner-only group setting, modeled after the existing allow_enterprise_bypass_placeholder_confirmation pattern.

Disabled by default to preserve current reassignment behavior. This commit only adds the opt-in flag; finder and authorization logic will read it in subsequent changes.

References

#619432 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. If not already set, export GITLAB_SIMULATE_SAAS=1 and restart the Rails server.
2. In rails console, enable the feature flag:

   Feature.enable(:allow_non_enterprise_placeholder_reassignment)

3. Navigate to a top-level group where you are an Owner and the :domain_verification licensed feature is available.
4. Visit the group's General Settings page at /groups/<group-full-path>/-/edit.
5. Locate the "Allow reassigning placeholder users to non-enterprise members" checkbox under "Permissions and group features".
6. Toggle the checkbox and click Save.
7. Verify the setting persisted in rails console:

   Group.find_by_full_path('<group-full-path>').namespace_settings.allow_non_enterprise_placeholder_reassignment?

8. Disable the feature flag to confirm it gates checkbox visibility:

   Feature.disable(:allow_non_enterprise_placeholder_reassignment)

9. Refresh the group settings page and confirm the checkbox has disappeared.

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