[6000.1.13] 'CoreKeywordDescriptors' does not contain a definition for 'ForwardPlus'

Some of the internals have changed, and we have this compiler error after upgrading to `6000.1.13f1`:

```
com.zallist.universal-shadergraph-extensions@94d5546221e8\Editor\ShaderGraph\Targets\UniversalSimpleLitSubTarget.cs(876,42): 
error CS0117: 'CoreKeywordDescriptors' does not contain a definition for 'ForwardPlus'
```