Adds tickets_confidential_by_default to controller and update service

What does this MR do and why?

Contributes to Customize Service Desk issue confidentiality (#33091 - closed)

❗ This feature is hidden behind the feature flag 🎏 service_desk_tickets_confidentiality.

We received feedback that the first action agents perform on a Service Desk ticket is to mark it as non-confidential. We already added the DB field and logic in Select default Service Desk ticket visibility (!150025 - merged). This MR adds the tickets_confidential_by_default field to ServiceDeskController and ServiceDeskSettings::UpdateService. It also pushes the feature flag to the frontend on the project settings page.

Planned MRs for this feature

1. ✔ Add DB field and logic
2. 🎯 Add field to API controller
3. Add field to frontend Service Desk settings

MR acceptance checklist

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Screenshots or screen recordings

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🚫 backend only

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

I added steps to validate ServiceDeskSettings::UpdateService locally. If you'd like to have additional steps, please leave a comment.

1. Select a project

   project = Project.find(7)

2. Now enable the setting

   ServiceDeskSettings::UpdateService.new(project, User.first, tickets_confidential_by_default: false).execute

3. Check whether the setting has been changed. It should be true (default), because the feature flag is disabled

   project.reset.service_desk_setting.tickets_confidential_by_default

4. Now enable the feature flag

   Feature.enable(:service_desk_tickets_confidentiality, project)

5. Repeat steps 2) and 3). The output should be false now.
6. (Optional) Use this to rollback the changes to your project

   ServiceDeskSettings::UpdateService.new(project, User.first, tickets_confidential_by_default: true).execute
   Feature.disable(:service_desk_tickets_confidentiality, project)