net/probes: Certificate expiry cannot advance on reload

[BenPope]

Version & Environment

Redpanda version: v23.3.1+

What went wrong?

Reloading a certificate doesn't advance the expiry time.

What should have happened instead?

Metric should update to the new expiry.

How to reproduce the issue?

1. Start Redpanda with a certificate (ca or server) that expires at x
2. Replace the certificate with an expiry of y > x
3. Observe metric says that expiry is still x

Fix the code

_cert_expiry_time and _ca_expiry_time should be reset near the beginning, but it should also be fail safe, so the instinctive choice of setting it to time_point::max() is probably not ideal.