refactor: replace QDateTime::fromTime_t with QDateTime::fromSecsSinceEpoch

Summary:
This is a backport of core-gui#349

Depends on D18183

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18184