qmake should enable c++14, c++17 by default

Qt 6 requires a C++17 compiler. However, qmake for user applications still only enables C++11 by default:

warning(CONFIG =$$CONFIG)
[...]
CONFIG =lex yacc debug exceptions depend_includepath testcase_targets import_plugins import_qpa_plugin windows file_copies qmake_use qt warn_on release link_prl flat debug_and_release debug_and_release_target precompile_header autogen_precompile_source embed_manifest_dll embed_manifest_exe shared plugin_manifest win32 msvc copy_dir_files release ReleaseBuild Release build_pass entrypoint c++11 debug qml_debug release ReleaseBuild Release build_pass c++11 console

It should arguably also enable C++14, C++17 by default.