FIX - RelativeCursorPosition Changed<> query filter (#20102)

## Problem

This pseudocode was triggering constantly, even with the Changed<> query
filter.

```rust
pub fn check_mouse_movement_system(
    relative_cursor_positions: Query< &RelativeCursorPosition, (Changed<RelativeCursorPosition>, With<Button>)>,
) {}
```

## Solution

- Added a check to prevent updating the value if it hasn't changed.

---------

Co-authored-by: Giacomo Stevanato <giaco.stevanato@gmail.com>

crates/bevy_ui/src/focus.rs

@@ -267,7 +267,9 @@ pub fn ui_focus_system(
             // Save the relative cursor position to the correct component
             if let Some(mut node_relative_cursor_position_component) = node.relative_cursor_position
             {
-                *node_relative_cursor_position_component = relative_cursor_position_component;
+                // Avoid triggering change detection when not necessary.
+                node_relative_cursor_position_component
+                    .set_if_neq(relative_cursor_position_component);
             }
 
             if contains_cursor {