Remove gym dependency

[saulfield]

More progress towards #1981.

• Fix make test
• Change all remaining references of gym to gymnasium
• Remove hiway-v0 env
• Remove gym v21 compatibility wrapper
• Remove gym dependency
• Made gymnasium dependency optional

[saulfield]

All the examples appear to be working with gymnasium aside from drive and platoon. There appears to be an issue with stable_baselines3 (see the failing CI test). I tried using v2.0.0 which added support for gymnasium, but the error is still happening. I've made this a draft PR since I think we should just do this all in one go, instead of leaving gym as a dependency for the rl examples, which would require some additional code changes.

[Adaickalavan]

PR #2075 updates the drive and platoon examples to use Stable Baselines3 v2.0.0 and gymnasium.

[Gamenot]

Where did this come from?

[saulfield]

I remember adding it to get a test to pass. I'll try removing it.