www/qt5-webengine: Update to 5.15.8
- Under the hood runs Chromium 87.0.4280.144 with security fixes up to version 96.0.4664.110 and patches were taken from of www/chromium 87.0.4280.141 (391acded85d8).
- Prune some unneeded patches (mostly from the headless and unittest areas) and try to align the unbundling of dependencies similar to qt6-webengine for consistency.
- Adjust Mk/Uses/qt-dist.mk accordingly as the naming of the distfiles has changed since Qt 5.15.3.
Tested by: tcberner
Differential Revision: https://reviews.freebsd.org/D38165