Qt5: Update to 5.15.15

Update devel/pyside2* and devel/shiboken2 to 5.15.15

Update www/qt5-webengine to include the latest security fixes in
the chromium submodule

Release note: https://code.qt.io/cgit/qt/qtreleasenotes.git/about/qt/5.15.15/release-note.md

MFH: 2024Q3
Security: 66907dab-6bb2-11ef-b813-4ccc6adda413
(cherry picked from commit 9e1e9e17c5920878facfea419e9ec1d1005ef189)