Fix some friend access violations by allowing others access. (#8594)

Rename Friend attribute to Access attribute.
Updates submodule to v0.21.0.0 as well.

Content.Shared/Flash/SharedFlashableComponent.cs

@@ -3,7 +3,7 @@ using Robust.Shared.Serialization;
 
 namespace Content.Shared.Flash
 {
-    [NetworkedComponent, Friend(typeof(SharedFlashSystem))]
+    [NetworkedComponent, Access(typeof(SharedFlashSystem))]
     public abstract class SharedFlashableComponent : Component
     {
         public float Duration { get; set; }