Modernise and clean up the wording of the Debian welcome message

Replace PGP/GPG with OpenPGP as that is the name of the standard.

Drop mention of telnet/rlogin as those are obsolete system access protocols.

Mention that publickey for SSH is required since passwords are insecure.

De-prioritise looking up SSH fingerprints on the web since it is tedious.

Mention that the SSH known_hosts file can be downloaded via SSH too.

Mention that the SSH fingerprints can be looked up using DNSSEC/SSHFP.

Switch from mentioning SSL to TLS since the former is obsolete and unused.

Mention that almost all debian.org servers now have TLS.

Mention that some Debian websites now have Tor Onion Services.

Mention that SFTP/rsync can also be used when uploading over SSH.

Since Salsa dropped the -guest suffix, just refer to them as guest accounts.

Mention the OFTC IRC network instead of irc.debian.org, which breaks TLS.