[Port] [6000.0] Remove vector truncation warning for SSAO

Fix shader warnings in URP ScreenSpaceAmbientOcclusion.shader.

> Shader warning in 'Hidden/Universal Render Pipeline/ScreenSpaceAmbientOcclusion': implicit truncation of vector type at Packages/com.unity.render-pipelines.universal/Shaders/Utils/ScreenSpaceAmbientOcclusion.shader(211) (on metal)

```
                half4 FragGaussianAfterOpaque(Varyings input) : SV_Target
                {
                    half ao = VerticalGaussianBlur(input);
                    return half4(0.0, 0.0, 0.0, ao);
                }
```

VerticalGaussianBlur() returns `half4(ao, ao, ao, 1)` so the fix is to simply pick the r component.

```
                half4 FragGaussianAfterOpaque(Varyings input) : SV_Target
                {
                    half ao = VerticalGaussianBlur(input).r;
                    return half4(0.0, 0.0, 0.0, ao);
                }
```

Author: svc-reach-platform-support

Packages/com.unity.render-pipelines.universal/Shaders/Utils/ScreenSpaceAmbientOcclusion.shader

@@ -164,7 +164,7 @@ Shader "Hidden/Universal Render Pipeline/ScreenSpaceAmbientOcclusion"
 
                 half4 FragGaussianAfterOpaque(Varyings input) : SV_Target
                 {
-                    half ao = VerticalGaussianBlur(input);
+                    half ao = VerticalGaussianBlur(input).r;
                     return half4(0.0, 0.0, 0.0, ao);
                 }
 
@@ -208,7 +208,7 @@ Shader "Hidden/Universal Render Pipeline/ScreenSpaceAmbientOcclusion"
 
                 half4 FragKawaseAfterOpaque(Varyings input) : SV_Target
                 {
-                    half ao = KawaseBlur(input);
+                    half ao = KawaseBlur(input).r;
                     return half4(0.0, 0.0, 0.0, ao);
                 }