Set QMAKE_CXXFLAGS_CXX2A variable to -std:c++20 while building projects
with qmake and MSVC 16.11 or greater (_MSC_FULL_VER >= 192930133), when
c++2a is specified in the project config.
Pick-to: 6.2 6.3
Change-Id: If9a48d1ed16cb05c5cc01414ce8a4344e22438fe
Reviewed-by: Kai Koehne <kai.koehne@qt.io>