change the default width and height of Size to Val::Auto (#7475)

# Objective

In CSS Flexbox width and height are auto by default, whereas in Bevy their default is `Size::Undefined`.
This means that, unlike in CSS, if you elide a height or width value for a node it will be given zero length (unless it has an explicitly sized child node). This has misled users into falsely assuming that they have to explicitly set a value for both height and width all the time.

relevant issue: #7120

## Solution

Change the `Size` `width` and `height` default values to `Val::Auto`

## Changelog

* Changed the `Size` `width` and `height` default values to `Val::Auto`

## Migration Guide

The default values for `Size` `width` and `height` have been changed from `Val::Undefined` to `Val::Auto`.
It's unlikely to cause any issues with existing code.

crates/bevy_ui/src/geometry.rs

@@ -339,7 +339,7 @@ pub struct Size {
 }
 
 impl Size {
-    pub const DEFAULT: Self = Self::all(Val::DEFAULT);
+    pub const DEFAULT: Self = Self::all(Val::Auto);
 
     /// Creates a new [`Size`] from a width and a height.
     ///