Xen 4.21 release schedule

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

Hello everyone,

As there hasn't been any feedback on the proposed timeline shared here:
 https://lore.kernel.org/xen-devel/666e3f49-2f92-4828-8897-8579832bcaa2@xxxxxxxxx/T/#u

We can proceed with the suggested timeline as the release schedule for 
Xen 4.21.

I've updated the Xen 4.21 Release Notes with this schedule, which can now be
accessed via the RN link:
   https://xenbits.xen.org/docs/unstable/support-matrix.html

Best regards,
 Oleksii