[RLlib] Actually bump gym dep

[smorad]

Why are these changes needed?

I previously made rllib work with gym==0.24.0 in #26190, bumping the gym version in requirements.txt. Unfortunately, it seems like the version is hardcoded in setup.py, which I missed. This commit should make pip install gym==0.24.0 ray[rllib] run without error.

Related issue number

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• 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 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 :(

[avnishn]

@smorad there's a few other places in the library you'll need to update this version dependency.

[avnishn]

closing in favor of @sven1977's gym pr #28369