replace outdated interface

[davidklaftenegger]

cryptography-42.0.0 introduces new interfaces to query certificate lifetime. The only difference is that the UTC timezone is set in the datetime object, instead of the timezone being undefined. However, the old interface now prints deprecation warnings.

[moepman]

We should use this interface on distributions that ship such a recent version of cryptography - however we can not drop the old code path as long as supported distributions are still shipping older versions.