certs: early renew long-lived certificates

certs: early renew long-lived certificates

if our self-signed certificate expires in more than 825 days, but was
created after July 2019 it won't be accepted by modern Apple devices. we
fixed the issuance to generate shorter-lived certificates in November
2019, this cleans up the existing ones to fix this and similar future
issues.

two years / 730 days as cut-off was chosen since it's our new maximum
self-signed certificate lifetime, and should thus catch all old-style
certificates.

another positive side-effect is that we can now phase out support for
older certificates faster, e.g. if we want to move to bigger keys,
different signature algorithms, or anything else in that direction.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>