Increase default normal bias to avoid common artifacts (#10346)

# Objective

Bevy's default bias values for directional and spot lights currently
cause significant artifacts. We should fix that so shadows look good by
default!

This is a less controversial/invasive alternative to #10188, which might
enable us to keep the default bias value low, but also has its own sets
of concerns and caveats that make it a risky choice for Bevy 0.12.

## Solution

Bump the default normal bias from `0.6` to `1.8`. There is precedent for
values in this general area as Godot has a default normal bias of `2.0`.

### Before


![image](https://github.com/superdump/bevy/assets/2694663/a5828011-33fc-4427-90ed-f093d7389053)

### After


![image](https://github.com/bevyengine/bevy/assets/2694663/0f2b16b0-c116-41ab-9886-1ace9e00efd6)

## Migration Guide

The default `shadow_normal_bias` value for `DirectionalLight` and
`SpotLight` has changed to accommodate artifacts introduced with the new
shadow PCF changes. It is unlikely (especially given the new PCF shadow
behaviors with these values), but you might need to manually tweak this
value if your scene requires a lower bias and it relied on the previous
default value.

Author: cart

crates/bevy_pbr/src/light.rs

@@ -116,7 +116,7 @@ pub struct SpotLight {
 
 impl SpotLight {
     pub const DEFAULT_SHADOW_DEPTH_BIAS: f32 = 0.02;
-    pub const DEFAULT_SHADOW_NORMAL_BIAS: f32 = 0.6;
+    pub const DEFAULT_SHADOW_NORMAL_BIAS: f32 = 1.8;
 }
 
 impl Default for SpotLight {
@@ -210,7 +210,7 @@ impl Default for DirectionalLight {
 
 impl DirectionalLight {
     pub const DEFAULT_SHADOW_DEPTH_BIAS: f32 = 0.02;
-    pub const DEFAULT_SHADOW_NORMAL_BIAS: f32 = 0.6;
+    pub const DEFAULT_SHADOW_NORMAL_BIAS: f32 = 1.8;
 }
 
 /// Controls the resolution of [`DirectionalLight`] shadow maps.