Remove computers from both lookups when they timeout

In 5d8c46c7e6, we switched to using UUIDs
for looking up computers (rather than an integer ID). However, for
compatibility in some of the command code, we need to maintain the old
integer lookup map.

Most of the code was updated to handle this, *except* the code to remove
a computer from the registry. This meant that we'd fail to remove a
computer from the UUID lookup map, so computers ended up in a phantom
state where they were destroyed, but still accessible.

This is not an issue on 1.20.4, because the legacy int lookup map was
removed.

Fixes #1760

projects/common/src/main/java/dan200/computercraft/shared/computer/core/ServerComputerRegistry.java

@@ -42,13 +42,14 @@ public class ServerComputerRegistry {
     }
 
     void update() {
-        var it = getComputers().iterator();
+        var it = computersByInstanceId.values().iterator();
         while (it.hasNext()) {
             var computer = it.next();
             if (computer.hasTimedOut()) {
                 computer.unload();
                 computer.onRemoved();
                 it.remove();
+                computersByInstanceUuid.remove(computer.getInstanceUUID());
             } else {
                 computer.tickServer();
             }