Change Service Desk native attachments docs for GA in 15.10

What does this MR do and why?

15.8 introduced native email attachments for Service Desk emails behind the FF service_desk_new_note_email_native_attachments. For 15.10 we aim to release that feature in general availability. This MR changes the Service Desk docs portion for this feature, so it's clear that it's not behind a FF any more.

• Issue: Native attachments for Service Desk emails (#11733 - closed)
• FF default enabled MR: Native attachments for Service Desk emails (!114550 - merged)
• Release post item MR: Native attachments for Service Desk emails (gitlab-com/www-gitlab-com!121161 - merged)
• FF rollout issue: [Feature flag] Rollout of `service_desk_new_not... (#386860 - closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.