kconfig: qconf: replace deprecated QString::sprintf() with QTextStream

QString::sprintf() is deprecated in the latest Qt version, and spawns
a lot of warnings:

  HOSTCXX scripts/kconfig/qconf.o
scripts/kconfig/qconf.cc: In member function ‘void ConfigInfoView::menuInfo()’:
scripts/kconfig/qconf.cc:1090:61: warning: ‘QString& QString::sprintf(const char*, ...)’ is deprecated: Use asprintf(), arg() or QTextStream instead [-Wdeprecated-declarations]
 1090 |      head += QString().sprintf("<a href=\"s%s\">", sym->name);
      |                                                             ^
In file included from /usr/include/qt5/QtGui/qkeysequence.h:44,
                 from /usr/include/qt5/QtWidgets/qaction.h:44,
                 from /usr/include/qt5/QtWidgets/QAction:1,
                 from scripts/kconfig/qconf.cc:7:
/usr/include/qt5/QtCore/qstring.h:382:14: note: declared here
  382 |     QString &sprintf(const char *format, ...) Q_ATTRIBUTE_FORMAT_PRINTF(2, 3);
      |              ^~~~~~~
scri...