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/Movement/Components/MovementSpeedModifierComponent.cs

@@ -4,7 +4,7 @@ using Robust.Shared.GameStates;
 namespace Content.Shared.Movement.Components
 {
     [RegisterComponent]
-    [NetworkedComponent, Friend(typeof(MovementSpeedModifierSystem))]
+    [NetworkedComponent, Access(typeof(MovementSpeedModifierSystem))]
     public sealed class MovementSpeedModifierComponent : Component
     {
         public const float DefaultBaseWalkSpeed = 3.0f;