Shadergraph Terrain samples don't support more than 4 layers in URP.

Issue from the following forum thread : https://discussions.unity.com/t/custom-urp-terrain-shader-add-more-than-4-layers/1713788

Steps to reproduce:

1. Import shadergraph terrain samples
2. Try to add more than 4 layers in the "CNM simple blend" sample with the already existing method
3. Try to paint a new layer

Actual results: 

Only the new layer will be displayed

Expected results: 

The new layer stacks up properly on the terrain

Reproducible with versions: 6.3+

Not reproducible with versions: 

Can’t test with versions: 

Tested on (OS): Windows

Notes:

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