Qt5: Update to 5.15.19

This is the final release of Qt5. If your port still uses Qt5 and could
possibly use Qt6 instead, now would be a good time to make the switch.

PySide2: Update to 5.15.19

Announcement:
https://lists.qt-project.org/pipermail/announce/2026-May/000626.html
Release notes:
https://code.qt.io/cgit/qt/qtreleasenotes.git/tree/qt/5.15.19/release-note.md