feat: Add audit event for group Pages public access control setting

What does this MR do and why?

Toggling the "Remove public access" setting (force_pages_access_control) at the group level previously produced no audit event, creating a gap in audit coverage for a security-relevant setting that affects all projects within a group and its subgroups.

This MR adds the group_force_pages_access_control_updated audit event by:

1. Creating a new audit event type YAML file at ee/config/audit_events/types/group_force_pages_access_control_updated.yml
2. Adding force_pages_access_control to EVENT_NAME_PER_COLUMN in Namespaces::NamespaceSettingChangesAuditor, so changes to this column are audited consistently with other namespace settings
3. Updating the spec to add a test case for the new column and removing it from the denylist
4. Updating the audit event types documentation

This is consistent with how project-level Pages access changes are audited via Projects::ProjectFeatureChangesAuditor (project_feature_pages_access_level_updated).

References

• Closes #595288 (closed)
• Reference: existing project-level audit event ee/config/audit_events/types/project_feature_pages_access_level_updated.yml

Screenshots or screen recordings

No UI changes — this is a backend-only change adding audit instrumentation.

Before	After
No audit event when toggling "Remove public access" at group level	group_force_pages_access_control_updated audit event created

How to set up and validate locally

1. Toggle the Remove public access setting for your group.
2. Check Group Audit events — a group_force_pages_access_control_updated event should appear

MR acceptance checklist

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