Fully migrate from `gym` to `gymnasium`

[pminervini]

Some parts of the project still depend on gym rather than its newer drop-in replacement gymnasium, can I do a pull request to fix that?

[puyuan1996]

Hello, thank you very much for your proposal!

If you are interested in taking this on, we would be honored to have your contribution. Fully migrating the project to gymnasium is a significant improvement for us.

Please feel free to submit a Pull Request at your convenience. Should you have any questions or wish to discuss specific details during the process, please do not hesitate to reach out in this thread.

Thank you again for your generous support!