Allow dynamically configuring dropdown hint text

Used for "confine mouse mode" dropdown tooltip text.

osu.Game/Graphics/UserInterfaceV2/FormDropdown.cs

@@ -30,7 +30,11 @@ namespace osu.Game.Graphics.UserInterfaceV2
         /// <summary>
         /// Hint text containing an extended description of this slider bar, displayed in a tooltip when hovering the caption.
         /// </summary>
-        public LocalisableString HintText { get; init; }
+        public LocalisableString HintText
+        {
+            get => header.HintText;
+            set => header.HintText = value;
+        }
 
         /// <summary>
         /// The maximum height of the dropdown's menu.
@@ -46,7 +50,6 @@ namespace osu.Game.Graphics.UserInterfaceV2
             RelativeSizeAxes = Axes.X;
 
             header.Caption = Caption;
-            header.HintText = HintText;
         }
 
         protected override void LoadComplete()