DSNs work in Thunderbird 128

Remove the restriction on using Delivery Status Notifications with Send Later for Thunderbird 128 and newer, because they work now.

Note: Downloading and installing send_later_beta.xpi will subscribe you to future beta releases. We love beta-testers! The advantage is that if there's a bug that affects your workflow you'll help find it and get it fixed quickly. The disadvantage is that bugs are a bit more likely in beta releases. You can unsubscribe from beta releases at any time by downloading and installing send_later.xpi or installing from addons.thunderbird.net.