Update script for certbot installed via snapd

[maxxer]

• Remove certbot-auto support (see #124) or add a warning it's being deprecated?
• Systemd timer name has changed (snap.certbot.renew.timer)

If anything else comes to your mind @jjakob

[jjakob]

• Remove certbot-auto support (see #124) or add a warning it's being deprecated?

I'd move it down the priority list (it should be enough to swap it in the "which" command), not remove it as some people may still use certbot-auto until it stops working. certbot-auto gives a warning itself when the script runs it so I don't think there's a need to add it.

• Systemd timer name has changed (snap.certbot.renew.timer)

Possibly only for certbot installed via snaps?

If anything else comes to your mind @jjakob

Not really, it's running stable for me. Other things are already open in other issues.

[maxxer]

superseded by #132