Fix: Clears directional navigation map between rebuilds

[kfc35]

Objective

• Closes DirectionalNavigation: auto_rebuild_ui_navigation_graph does not clear map, leaves stale edges after re-renders #21949

Solution

• Implements the solution as suggested by 21949. The Directional Navigation Map does not prune any old edges that may be connected to removed Nodes, so we clear the map between rebuilds of the map.

(If the more preferable solution figures out what nodes were removed and then calls map.remove_multiple instead, you can feel free to reject this)

Testing

• Did you test these changes? If so, how?
  I did not test this change, but the reporter of the issue did (via backport to an older version of Bevy). However, the fix is straightforward and explains itself. If you would like an automated test somehow, some pointers to tests that do similar would be welcome.

[alice-i-cecile]

A small note in the documentation about this behavior might be nice, but it also feels unsurprising that this system does this so I won't block.

[viridia]

I'm fine with this.

If you are concerned about the overhead of rebuilding the map each time, then a mark-and-sweep approach might work.