[Qt] Fix deprecated QDateTime(const QDate &)

Summary:
Qt 5.15 has deprecated the construction of a QDateTime object from a
QDate. This was used in our codebase to build a QDateTime
corresponding to the start of the day for the given QDate, but Qt
introduced a dedicated method for this use case since 5.14.

Test Plan:
With Qt 5.5.1 and Qt 5.15:

ninja all check

Run bitcoin-qt and check the transaction view filter still works as
expected (including with an arbitrary range input).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6575