community/qt5-qtwebengine: upgrade to 5.15.11-lts

I made an error earlier, this is the actual, tagged, 5.15.11 release. When moving from 5.15.3_git<commit date> I looked at MODULE_VERSION, that is bumped right after a release is tagged, and also still stayed with fetching git commit tar balls.

I've also missed the latest 87-based branch chromium security fix https://invent.kde.org/qt/qt/qtwebengine-chromium/-/commit/be349eaf62e77955791d7bf29f893f1e8a37daf4 not sure if there is an CVE for that.