Drop the hard requirement of OpenPGP communication for mirror operators.

This change doesn't imply any regression, compared to the current state
of things, given we've already somewhat given up on this in practices
for a long time (the work required to maintain the corresponding OpenPGP
keyring was too much compared to the perceived advantage).

See the "[RFC] Dropping requirement for OpenPGP communication with HTTP
mirror operators?" thread on tails-dev@, for details.

wiki/src/contribute/how/mirror.mdwn

@@ -71,7 +71,7 @@ You will also need around **5-10 GiB** of disk space.
 If you satisfy these practical requirements, please read on!
 Else, please consider seeding Tails images over BitTorrent instead.
 
-Before starting doing any real work on this topic, please get in touch
+Before starting doing any real work on this topic, you may want to get in touch
 (<tails-mirrors@boum.org>, [[OpenPGP key|doc/about/openpgp_keys#mirrors]]) and send us
 your OpenPGP public key, so that any further communication between us
 can be properly encrypted and authenticated.