Drop supporting CUDA 11.8 in favor of 12.6 (#148)

Summary:
This PR aims to reduce maintenance effort now that the latest `conda-forge::pytorch` package [has dropped the CUDA 11.8 support](conda-forge/pytorch-cpu-feedstock#293 (comment)), and we no longer have any use cases for CUDA 11.8.

Additionally, removing the unnecessary use of 'pytorch' and 'nvidia' channels to simplify the number of conda channels used.

## Checklist:

- [x] Adheres to the [style guidelines](https://facebookincubator.github.io/momentum/docs/developer_guide/style_guide)
- [x] Codebase formatted by running `pixi run lint`

Pull Request resolved: #148

Test Plan: CI

Reviewed By: nickyhe-gemini

Differential Revision: D66679272

Pulled By: jeongseok-meta

fbshipit-source-id: a3fb7ef53e2b62a501601d871c76b99315b3ae96

Author: jeongseok-meta