Don't use wxBORDER_SUNKEN for wxSpinCtrl in dark mode

This applies the change done in f5dd5eaf19 (Use darker border in wxMSW
dark mode by default, 2023-06-16) for all the other controls to
wxSpinCtrl too, as it still explicitly used wxBORDER_SUNKEN.

The border around the text part of the control is now correct, but it
looks even uglier in the dark mode due to a completely wrong border
being used around the spin button -- but this will be addressed by the
upcoming commits.

src/msw/spinctrl.cpp

@@ -283,7 +283,7 @@ bool wxSpinCtrl::Create(wxWindow *parent,
 
     // the border is only used for the text control part
     if ( (style & wxBORDER_MASK) == wxBORDER_DEFAULT )
-        style |= wxBORDER_SUNKEN;
+        style |= DoTranslateBorder(wxBORDER_THEME);
 
     SetWindowStyle(style);