[Tails-l10n] Release schedule for Tails 6.4

Hi,

groente will be the RM for Tails 6.4.

The current plan is:
- Monday, June 17: build images, start testing
- Tuesday, June 18: releasing

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

Cheers!