Fix link modal not shown after access upgrade (#12411)

* Fix link modal not shown after access upgrade

We dont show the modal since there was a mistake in the isRoomJoinable function.
It used a state variable instead of reacomputing the join rule with room.getJoinRule()
The state is a captured variable that from before the link share click -> does not update at that time.

* dont shadow var

src/components/views/rooms/RoomHeader/CallGuestLinkButton.tsx

@@ -79,7 +79,6 @@ export const CallGuestLinkButton: React.FC<{ room: Room }> = ({ room }) => {
                 // If the user cannot invite the Knocking is not given as an option.
                 canInvite,
             }).finished.then(() => {
-                // we need to use the function here because the callback got called before the state was updated.
                 if (isRoomJoinable()) showLinkModal();
             });
         }