REF: Rename mode.nullable_backend to mode.dtype_backend
From the dev call yesterday, it was discussed to make the new global configuration for 2.0 `mode.nullable_backend="pyarrow"|"pandas"` more generically named. I have changed it to `mode.dtype_backend` as we may decide to not always associate this option with the `use_nullable_dtype` keyword argument in the IO readers

cc @phofl @jrbourbeau 
@phofl any thoughts on `dtypes_backend` vs. `dtypes` vs. some other name?  The nice thing about the old name: It implies that you have to use nullable dtypes that the option can take effect. If you don't use nullable_dtypes then nullable_backend does not make much sense.

The new name implies that it can work globally, which is not possible right now. In general, I'd be ok with this, but we have to clarify in the docs that this works only in specific situations right now Agreed with the concern, at least for 2.0 where we will be paring this closely with nullable types, but I like that this change future-proofs this option to be more applicable to the non-nullable types in the future thx @mroeschke 