Prefer OffsetDateTime instead of ZonedDateTime in TUF

[vlsi]

Description

ZonedDateTime allows dates with named timezones (e.g. Europe/Berlin) that depend on the local timezone database.
I wonder if we really need to support named time zones.

It might be slightly easier to deal with OffsetDateTime that never depends on the timezone database, and which is never subject to DST, etc.

[michael-o]

I'd even say that an Instant will do the job in incur minimal overhead.

[loosebazooka]

@patflynn. I believe this was intentional to support some weirdness in the sigstore specific tuf root