Update the deb.tails.boum.org archive signing key (2021 edition)
221F9A3C6FA3E09E182E060BC7988EA7A358D82E expires in May.
Once this is done on our infra:
- submit a MR to update the public key in tails/tails
- file an issue to consider transitioning to a brand new key pair next year: at that time, our current key will be 10 years old; such a transition may require more advance planning and multiple steps