Pin to `dask>=2025.5.1`

Following on from #332, we can now pin to `dask>=2025.5.1`, implementing the move from the defunct `dask.base.collections_to_expr` to extent `dask.base.collections_to_expr`.

Note that 2025.5.1 is the latest Dask version, so we don't know if we need to pin a maximum version or not (hopefully not :))