depends: qt 5.15.14 and fix macOS build with Clang 18

Summary:

Also adds a patch to Qts internal libpng, to fix compilation using Clang 18, when targetting macOS.

Backport of core#30198 and core#30227.

Test Plan: Run the guix builds

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17265