Fix a bunch of compiler warnings (#10518)

Content.Client/Power/APC/ApcVisualizer.cs

@@ -15,6 +15,7 @@ namespace Content.Client.Power.APC
         public static readonly Color EmagColor = Color.FromHex("#1f48d6");
 
         [UsedImplicitly]
+        [Obsolete("Subscribe to your component being initialised instead.")]
         public override void InitializeEntity(EntityUid entity)
         {
             base.InitializeEntity(entity);
@@ -37,6 +38,7 @@ namespace Content.Client.Power.APC
             sprite.LayerSetShader(Layers.Environment, "unshaded");
         }
 
+        [Obsolete("Subscribe to AppearanceChangeEvent instead.")]
         public override void OnChangeData(AppearanceComponent component)
         {
             base.OnChangeData(component);