Mention that there are known fake Tails OpenPGP keys

https://tails.boum.org/doc/about/openpgp_keys/index.en.html should mention that there a known false keys.

Feature Branch: bitingbird:doc/7225-fake_keys

Related issues

• Related to #9112 (closed)

Original created by @BitingBird on 7225 (Redmine)