Reduce semi-opaque layers at song select

I made these changes while working on
https://github.com/ppy/osu/pull/30579. Basically, it's hard to fix the
ranks not loading while underneath the footer, and the transparency both
looks bad, and is going away in the redesign.

I've chosen values here that are moving *in the direction* of the new
design without overhauling everything.

- I know that there's still some transparency. I did this because it
helps keep all current elements / colours contrasting without too much
effort.
- I completely removed the transparency adjustments on the beatmap
panels. This always looked bad due to being applied per-layer, and I
don't think it added much.

osu.Game/Screens/Select/BeatmapCarousel.cs

@@ -1116,11 +1116,6 @@ namespace osu.Game.Screens.Select
             // adjusting the item's overall X position can cause it to become masked away when
             // child items (difficulties) are still visible.
             item.Header.X = offsetX(dist, visibleHalfHeight) - (parent?.X ?? 0);
-
-            // We are applying a multiplicative alpha (which is internally done by nesting an
-            // additional container and setting that container's alpha) such that we can
-            // layer alpha transformations on top.
-            item.SetMultiplicativeAlpha(Math.Clamp(1.75f - 1.5f * dist, 0, 1));
         }
 
         private enum PendingScrollOperation

osu.Game/Screens/Select/Carousel/DrawableCarouselItem.cs

@@ -86,8 +86,6 @@ namespace osu.Game.Screens.Select.Carousel
             };
         }
 
-        public void SetMultiplicativeAlpha(float alpha) => Header.BorderContainer.Alpha = alpha;
-
         protected override void LoadComplete()
         {
             base.LoadComplete();

osu.Game/Screens/Select/FilterControl.cs

@@ -97,8 +97,8 @@ namespace osu.Game.Screens.Select
             {
                 new Box
                 {
-                    Colour = Color4.Black,
+                    Colour = OsuColour.Gray(0.05f),
-                    Alpha = 0.8f,
+                    Alpha = 0.96f,
                     Width = 2,
                     RelativeSizeAxes = Axes.Both,
                 },

osu.Game/Screens/Select/Footer.cs

@@ -12,6 +12,7 @@ using osu.Framework.Graphics;
 using osu.Framework.Graphics.Containers;
 using osu.Framework.Graphics.Shapes;
 using osu.Framework.Input.Events;
+using osu.Game.Graphics;
 using osu.Game.Graphics.UserInterface;
 
 namespace osu.Game.Screens.Select
@@ -82,14 +83,15 @@ namespace osu.Game.Screens.Select
                 {
                     RelativeSizeAxes = Axes.Both,
                     Size = Vector2.One,
-                    Colour = Color4.Black.Opacity(0.5f),
+                    Colour = OsuColour.Gray(0.1f),
+                    Alpha = 0.96f,
                 },
                 modeLight = new Box
                 {
                     RelativeSizeAxes = Axes.X,
                     Height = 3,
                     Position = new Vector2(0, -3),
-                    Colour = Color4.Black,
+                    Colour = OsuColour.Gray(0.1f),
                 },
                 new FillFlowContainer
                 {