[Backend] Implement SetGroupValidityChecksService

Create Security::Configuration::SetGroupValidityChecksService that extends SetProjectSecuritySettingBaseService to handle enabling/disabling validity checks across all projects in a group.

• Purpose: Handle the core business logic for enabling/disabling validity checks across all projects in a group
• Responsibilities:
  • Query all projects within the group hierarchy
  • Filter out excluded projects from the update scope
  • Update the validity_checks_enabled setting for each project
  • Create audit logs for the group-wide configuration change
  • Handle post-update processing if needed