Stop recommending bcrypt everywhere

By passing no method to mkpasswd we make it select the strongest cipher
that libxcrypt recommends.

Replaces the example hashes with yescrypt hashes, which is the current
default.

docs/setup-example.nix

@@ -32,7 +32,7 @@
     x509.useACMEHost = config.mailserver.fqdn;
 
     # A list of all login accounts. To create the password hashes, use
-    # nix-shell -p mkpasswd --run 'mkpasswd -sm bcrypt'
+    # nix-shell -p mkpasswd --run 'mkpasswd -s'
     loginAccounts = {
       "user1@example.com" = {
         hashedPasswordFile = "/a/file/containing/a/hashed/password";