Use service_desk_settings.service_desk_enabled column instead of project.service_desk_enabled

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After all service_desk_settings.service_desk_enabled was populated with the correct value in Migrate data from `projects.service_desk_enable... (#412671) stop using project.service_desk_enabled column:

• Read only from service_desk_settings.service_desk_enabled column
• Write only to service_desk_settings.service_desk_enabled column
• Start ignoring projects.service_desk_enabled column https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#ignoring-the-column-release-m
• Review existing indexes for projects.service_desk_enabled
• Add indexes for new column if needed