[DOC]: `contributing.md` documents environment variables which are no longer in use

[dagardner-nv]

How would you describe the priority of this documentation request

Critical (currently preventing usage)

Please provide a link or source to the relevant docs

docs/source/developer_guide/contributing.md

Describe the problems in the documentation

I believe the following environment variables no longer have any effect:

• PYTHON_VER
• RAPIDS_VER
• TRITONCLIENT_VERSION
• CUDA_VER

Not only are these unsupported in the case of RAPIDS_VER the version number provided is incorrect.

(Optional) Propose a correction

No response

Code of Conduct

• I agree to follow this project's Code of Conduct
• I have searched the open documentation issues and have found no duplicates for this bug report