Avoid clearing chat overlay textbox when pressing "back" key binding

Generally this is expected behaviour for usages of focused text boxes
(ie. to clear search content), but not so much here.

Addresses https://github.com/ppy/osu/discussions/19403#discussioncomment-3230395.

osu.Game/Overlays/Chat/ChatTextBox.cs

@@ -13,6 +13,8 @@ namespace osu.Game.Overlays.Chat
 
         public override bool HandleLeftRightArrows => !ShowSearch.Value;
 
+        protected override bool ClearTextOnBackKey => false;
+
         protected override void LoadComplete()
         {
             base.LoadComplete();