old semantic layer specs showing on V2 docs

### Contributions

- [x] I have read the contribution docs, and understand what's expected of me.

### Link to the page on docs.getdbt.com requiring updates

https://docs.getdbt.com/docs/build/dimensions?version=2#categorical

### What part(s) of the page would you like to see updated?

I have V2 selected as the dbt version. In the Categorical dimension docs for V2 it says you can put `expr` under the `dimensions` block, but this is not possible in the new spec. To use `expr` when defining a dimension, it has to be under `derived_semantics`. The docs look like this:

But if you tried to do that in Studio when running 1.12 or Fusion/V2, you get an error:

Please remove the `expr` argument from the `dimensions` block and update the example to use a pre-defined column as a categorial dimension. This is confusing for customers and anyone else trying to learn how to use the semantic layer. It has come up as confusing for instructors and our customers during onboarding.

### Additional information

_No response_

Open the linked dimensions page with V2 selected and review the Categorical dimension section. Remove the expr argument from the dimensions block and update the example to use a pre-defined column as a categorical dimension. Done means the V2 documentation matches the current spec and no longer suggests the invalid configuration.

Written by the indexing model from the issue text.