[Tails-l10n] Release schedule for Tails 5.3

Hi,

Tails 5.3 should be released on August 9.

Dear manual testers, please let tails-rm@??? know how much
testing you can do:

- preferably on August 8 (afternoon/evening CEST)
- if needed, on August 9 (morning CEST)

Cheers!