Support `cuda-version` For DGL Conda Packages

## 🚀 Feature
Currently, DGL packages are separated by CUDA version using labels. i.e. to install DGL for CUDA 12.1, you use the `dglteam/label/cu121` channel and for CUDA 11.8 you use the `dglteam/label/cu118` channel. This is causing issues with RAPIDS CI where we test against several different CUDA versions, and are running into issues where we cannot resolve the right version of DGL because our dependency generator doesn't support setting a channel by CUDA version. Most other packages have a dependency on `cuda-version` that ensures the correct version of the package gets installed, so this is only an issue with DGL right now.

## Motivation

The RAPIDS Graph team has been overhauling our testing process for GNN packages (cuGraph-DGL, cuGraph-PyG, WholeGraph, etc.) and we want to expand CUDA 12 testing of our packages to align with the rest of RAPIDS. We have run into issues in CI where our automated process picks the wrong channel or package, and have resolved all of these problems with the exception of this issue for DGL.

## Alternatives

We might be able to make some changes to our dependency file generator to support this in the short-term, but in the long term we would like to rely on `cuda-version` which we do for all our other dependencies in conda.

## Pitch

We would like DGL to adopt dependency on `cuda-version` and remove the separate labels per CUDA version. So you could install for CUDA 12.1 by running `conda install -c conda-forge -c dglteam dgl cuda-version=12.1`

## Additional context

We're encountering this issue here: https://github.com/rapidsai/cugraph/pull/4346

@VibhuJawa @jakirkham