Update letsencrypt.inc.php to use Acme v2 if it is supported.

As per the discussion at #5030 (closed) which changes from:

					$letsencrypt_cmd = $letsencrypt . " certonly -n --text --agree-tos --expand --authenticator webroot --server https://acme-v01.api.letsencrypt.org/directory --rsa-key-size 4096 --email postmaster@$domain $cli_domain_arg --webroot-path /usr/local/ispconfig/interface/acme";

to:

					$letsencrypt_version = exec($letsencrypt . ' --version  2>&1', $ret, $val);
					if(preg_match('/^(\S+|\w+)\s+(\d+(\.\d+)+)$/', $letsencrypt_version, $matches)) {
						$letsencrypt_version = $matches[2];
					}
					if ($letsencrypt_version >=0.22) {
						$acme_version = 'https://acme-v02.api.letsencrypt.org/directory';
					} else {
						$acme_version = 'https://acme-v01.api.letsencrypt.org/directory';
					}
					$letsencrypt_cmd = $letsencrypt . " certonly -n --text --agree-tos --expand --authenticator webroot --server $acme_version --rsa-key-size 4096 --email postmaster@$domain $cli_domain_arg --webroot-path /usr/local/ispconfig/interface/acme";