Standardize on jiff as a Timestamp backend

[inklesspen]

Originally in #3484 I proposed adding jiff as a third backend, since the chrono maintainer has soft-deprecated chrono in favor of jiff.

I also noticed that time is still a required dependency even though by default it's not used: #3488.

In #3484, @GnomedDev said:

I dislike the situation currently where we have different backends for time, since it means we have non-additive features and implementation complexity. I feel like, since the Rust ecosystem seems to be moving to Jiff anyway, we should just take a stand and only support it.

I figured this would be best as a discussion thread for now.