Fix a bunch of compiler warnings (#10518)

Content.Client/PowerCell/PowerChargerVisualizer.cs

@@ -9,6 +9,7 @@ namespace Content.Client.PowerCell
     [UsedImplicitly]
     public sealed class PowerChargerVisualizer : AppearanceVisualizer
     {
+        [Obsolete("Subscribe to your component being initialised instead.")]
         public override void InitializeEntity(EntityUid entity)
         {
             base.InitializeEntity(entity);
@@ -23,6 +24,7 @@ namespace Content.Client.PowerCell
             sprite.LayerSetShader(Layers.Light, "unshaded");
         }
 
+        [Obsolete("Subscribe to AppearanceChangeEvent instead.")]
         public override void OnChangeData(AppearanceComponent component)
         {
             base.OnChangeData(component);