IPsec/OpenVPN: Use 4,096-bit RSA for host certificates as well

IPsec/OpenVPN: Use 4,096-bit RSA for host certificates as well

We already moved away from 2048-MODP in Core Update 170. Similarly,
German Federal Office for Information Security (BSI) recommends shifting
away from RSA keys below 3,000 bits by the end of 2022 at the latest.

The only place left in IPFire 2.x where we generate such keys is for
IPsec and OpenVPN host certificates. This patch increases their key
sizes to 4,096 bits as well - CA certificates already have this length.

Existing VPN connections cannot be migrated automatically. However, only
the respective host certificate has to be regenerated - thanks to the CA
certificates' key length being sufficient, there is no need to replace
the entire VPN CA.

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>