Update LW package version to 0.1.20

MaterialGraphProject/UnityPackageManager/manifest.json

 {
 	"registry": "https://staging-packages.unity.com",
 	"dependencies": {
-		"com.unity.render-pipelines.core" : "0.1.11",
-		"com.unity.render-pipelines.lightweight" : "0.1.11"
+		"com.unity.render-pipelines.lightweight" : "0.1.20"
 	}
 }