Add missing X-GitLab-ConfidentialIssue: header boolean to notes
Add missing X-GitLab-ConfidentialIssue: header boolean to notes
This MR is a follow-up to !101558 (merged) which was merged in 16.0 and added the new email header to issue notifications, 'X-GitLab-ConfidentialIssue' which will contain a boolean (true/false) value depending on the issue's confidentiality. That merge request missed notes replies (see #222908). This MR adds that missing header to notifications from notes, in the same way the original one does (it will default to false if the issue's confidentiality is nil.)
Related to #222908