Update the year in the About dialog

src/solvespace.cpp

@@ -1001,7 +1001,7 @@ void SolveSpaceUI::MenuHelp(Command id) {
 "law. For details, visit http://gnu.org/licenses/\n"
 "\n"
 "© 2008-%d Jonathan Westhues and other authors.\n"),
-PACKAGE_VERSION, 2020);
+PACKAGE_VERSION, 2021);
             break;
 
         default: ssassert(false, "Unexpected menu ID");