This command is using features that are not POSIX-compliant (the [[ and
the || in them), that despite being supported by many shells (including
bash) are not available in dash.

Since system() runs the code with /bin/sh that is an interface defined
as only being POSIX-compliant, with no extensions, it should not presume
a specific shell in that.

Closes: #6326
Signed-off-by: Mattia Rizzolo <mattia@mapreri.org>

Fixes ispconfig/ispconfig3#6061

- revert CAA checks from !1128 until we have a solid method for getting the correct CAA domain from a host name

Generate always new key 
use rsa 4096 like in certbot

Update letsencrypt.inc.php as per the discussion at #5030. Already pr'ed and merged in master via !778. Perhaps can be merged with stable too?