Fix: Clears directional navigation map between rebuilds (#22124)

# Objective

- Closes #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.

Author: kfc35

crates/bevy_input_focus/src/directional_navigation.rs

@@ -773,6 +773,8 @@ fn auto_rebuild_ui_navigation_graph(
         })
         .collect();
 
+    // clear the old nav map between rebuilds to ensure any removed entities' edges are pruned
+    directional_nav_map.clear();
     auto_generate_navigation_edges(&mut directional_nav_map, &nodes, &config);
 }