Qt6: Adapt to removed QMap functionality

QMap::iterator::operator+() was removed in 14090760a8, necessitating
extra code using std::next/prev to workaround.

QMap::unite is gone, the replacement QMap::insert was only introduced
in 5.15.

QMap key values need to have an operator==() available.

Task-number: QTCREATORBUG-24098
Change-Id: Ic4cf429ab18cad58b1218180de40eb65586afd77
Reviewed-by: Marco Bubke <marco.bubke@qt.io>

src/plugins/fakevim/fakevimplugin.cpp

@@ -697,8 +697,13 @@ void FakeVimExCommandsPage::apply()
         }
         settings->endArray();
         globalCommandMapping.clear();
+#if QT_VERSION >= QT_VERSION_CHECK(5, 15, 0)
+        globalCommandMapping.insert(defaultMap);
+        globalCommandMapping.insert(newMapping);
+#else
         globalCommandMapping.unite(defaultMap);
         globalCommandMapping.unite(newMapping);
+#endif
     }
 }
 
@@ -950,8 +955,13 @@ void FakeVimUserCommandsPage::apply()
         }
         settings->endArray();
         userMap.clear();
+#if QT_VERSION >= QT_VERSION_CHECK(5, 15, 0)
+        userMap.insert(dd->m_defaultUserCommandMap);
+        userMap.insert(current);
+#else
         userMap.unite(dd->m_defaultUserCommandMap);
         userMap.unite(current);
+#endif
     }
 }