[Doc] Add setuptools dependency for building docs

[peytondmurray]

Why are these changes needed?

This PR:

• Adds a temporary setuptools>=70.0.0 dependency for building the docs to make sphinxcontrib-redoc usable on Python 3.12. This is not intended to be a long term solution, so [Doc] Remove setuptools dependency; find upgrade path for sphinxcontrib-redoc #46374 has been made to track the follow-up work.
• Remove unused environment variable in the buildkite job that builds docs
• Bump the os version and python version used to build the docs on RTD

Related issue number

Closes #46205.

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(