depends: Qt 5.15.16

Summary:
Contains a handful of miscellaneous bug fixes.
We can drop a few of our patches.

Probably the last Qt5 bump before we need to use Qt6.

This is a backport of core#30774

Test Plan: guix build

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18136