Update the licensing info: Krita is effectively GPLv3-only

boud@thinkstation:~/dev/krita> git grep -i "version 3 as"
libs/ui/input/wintab/qxcbconnection.cpp: General Public License version 2.1 or version 3 as published by the Free
libs/ui/input/wintab/qxcbconnection_xi2.cpp: General Public License version 2.1 or version 3 as published by the Free
libs/ui/input/wintab/qxcbconnection_xi2.h: General Public License version 2.1 or version 3 as published by the Free
libs/ui/qtlockedfile/qtlockedfile.cpp: General Public License version 2.1 or version 3 as published by the Free
libs/ui/qtlockedfile/qtlockedfile.h: General Public License version 2.1 or version 3 as published by the Free
libs/ui/qtlockedfile/qtlockedfile_unix.cpp: General Public License version 2.1 or version 3 as published by the Free
libs/ui/qtlockedfile/qtlockedfile_win.cpp: General Public License version 2.1 or version 3 as published by the Free
libs/ui/qtsingleapplication/qtlocalpeer.cpp: General Public License version 2.1 or version 3 as published by the Free
libs/ui/qtsingleapplication/qtlocalpeer.h: General Public License version 2.1 or version 3 as published by the Free
libs/ui/qtsingleapplication/qtsingleapplication.cpp: General Public License version 2.1 or version 3 as published by the Free
libs/ui/qtsingleapplication/qtsingleapplication.h:** General Public License version 2.1 or version 3 as published by the Free

Or V3+

git grep -i "version 3 of"

libs/ui/input/wintab/kis_tablet_support_win_p.h: * the Free Software Foundation; either version 3 of the License, or
libs/widgetutils/kis_action_registry.cpp: * the Free Software Foundation; either version 3 of the License, or
libs/widgetutils/kis_action_registry.h: * the Free Software Foundation; either version 3 of the License, or
libs/widgetutils/xmlgui/KisShortcutsDialog_p.cpp: * the Free Software Foundation; either version 3 of the License, or
libs/widgetutils/xmlgui/KisShortcutsEditor_p.cpp: * the Free Software Foundation; either version 3 of the License, or
libs/widgetutils/xmlgui/KisShortcutsEditor_p.h: * the Free Software Foundation; either version 3 of the License, or
libs/widgetutils/xmlgui/kshortcutschemeseditor.h: * the Free Software Foundation; either version 3 of the License, or
plugins/tools/selectiontools/kis_selection_modifier_mapper.cc: * version 3 of the License, or (at your option) any later version.
plugins/tools/selectiontools/kis_selection_modifier_mapper.h: * version 3 of the License, or (at your option) any later version

Effectively, this means that Krita as a whole is now licensed under
GPLv3 only, until we remove the bits forked from Qt; from that moment
on, Krita as a whole is GPLv3+. The GPLv3+ code is the code contributed
by abrahams, who hasn't been around for a long time now.