Implement the service to set group validity checks

What does this MR do and why?

Adds Security::Configuration::SetGroupValidityChecksService to enable/disable validity checks across all projects within a group hierarchy.

Changelog: added
EE: true

Implementation details

• Extends SetProjectSecuritySettingBaseService following existing patterns
• Uses Gitlab::Database::NamespaceProjectIdsEachBatch for efficient project querying
• Updates validity_checks_enabled setting for projects in scope
• Filters out excluded projects from updates
• Creates audit logs including excluded projects

References

#570740 (closed)

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