[Tails-l10n] Release schedule for Tails 5.21

Hi,

intrigeri will be the RM for Tails 5.21

The current plan is:
- Thursday, December 21: build images, start testing
- Friday, December 22: releasing

@testers, please let the RM know how much time you'll have for manual QA
from Thursday 17:00 to Friday 11:00 (Europe/Berlin)

Cheers!