Qt5: Update to 5.15.18

This is the penultimate release of Qt5. While Qt5 is EOL, due to the
LGPL releases being a year behind the commercial releases, there will
still be one more patch release in May 2026.

PySide2: Update to 5.15.18

Announcement:
https://lists.qt-project.org/pipermail/announce/2025-October/000592.html
Release notes:
https://code.qt.io/cgit/qt/qtreleasenotes.git/tree/qt/5.15.18/release-note.md

MFH: 2025Q4