[release/air] Upgrade release tests that depend on upgraded modin to py38

[justinvyu]

Why are these changes needed?

https://github.com/ray-project/ray/pull/30895 upgraded the pinned version of modin, removing support for python <= 3.7. All the release tests run with py37 by default, so the cluster env was failing to build due to no compatible modin version. This PR runs these release tests with py38 instead, which aligns with the us moving to testing with py38 for all of CI.

Related issue number

Closes https://github.com/ray-project/ray/issues/36299

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 :(

[justinvyu]

Release tests: https://buildkite.com/ray-project/release-tests-pr/builds/42162#

[justinvyu]

Failing release test needs this issue to be solved still, but it's failing for other reasons. See this thread: https://github.com/ray-project/ray/issues/36254