DolphinQt: Resolve deprecated usage of margin()

This function has been marked as obsolete. In Qt 6.0 it's removed
entirely, so we must use getContentsMargin() explicitly instead
(margin() would do this for us).

Ditto for setMargin(), in which case we use setContentsMargin instead.
setMargin() would just pass its argument to all four parameters of
setContentsMargin(), so we can do the same.

Source/Core/DolphinQt/MainWindow.cpp

@@ -654,7 +654,7 @@ void MainWindow::ConnectStack()
 
   layout->addWidget(m_game_list);
   layout->addWidget(m_search_bar);
-  layout->setMargin(0);
+  layout->setContentsMargins(0, 0, 0, 0);
 
   connect(m_search_bar, &SearchBar::Search, m_game_list, &GameList::SetSearchTerm);
 

Source/Core/DolphinQt/QtUtils/FlowLayout.cpp

@@ -151,7 +151,11 @@ QSize FlowLayout::minimumSize() const
   for (const auto& item : m_item_list)
     size = size.expandedTo(item->minimumSize());
 
-  size += QSize(2 * margin(), 2 * margin());
+  // Any direction's margin works, as they all set the same within the constructor.
+  int margin = 0;
+  getContentsMargins(&margin, nullptr, nullptr, nullptr);
+
+  size += QSize(2 * margin, 2 * margin);
   return size;
 }
 

Source/Core/DolphinQt/QtUtils/WrapInScrollArea.cpp

@@ -48,7 +48,7 @@ void WrapInScrollArea(QWidget* parent, QLayout* wrapped_layout, QWidget* to_resi
 
   auto* scroll_layout = new QVBoxLayout;
   scroll_layout->addWidget(scroll_area);
-  scroll_layout->setMargin(0);
+  scroll_layout->setContentsMargins(0, 0, 0, 0);
 
   parent->setLayout(scroll_layout);
 }