Fix internal note warnings on service desk issues

What does this MR do and why?

When replying to internal notes in service desk issues, we should not show the warning about notifying email participants.

This also fixes the placeholder text that we show

References

Related to #399828 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create a service desk issue from the console:

   project = Project.find_by_full_path('flightjs/flight')
   FactoryBot.create(:issue, project: project, author: Users::Internal.support_bot, service_desk_reply_to: 'service.desk@example.com', confidential: true)

2. Open the ticket in the UI and create an internal note.

3. Add an email participant with the quick action: /add_email service.desk@example.com

4. Login as a different user and try to reply to the internal note

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