Align core model schema variable names to LayerStructure names

**Is your feature request related to a problem? Please describe.**

The variable names in `[core.layers]` do not align with the updated `LayerStructure` attributes (`soil_layers` -> `soil_layer_depths`, `canopy_layers` -> `n_canopy_layers`).

**Describe the solution you'd like**

Update the schema and tests to use consistent names.

Locate the [core.layers] schema, the LayerStructure definition, and the tests covering these attributes. Compare the current soil_layers and canopy_layers names with soil_layer_depths and n_canopy_layers; done means the schema and tests use the consistent names and pass.

Written by the indexing model from the issue text.