Fix unit1 bug

[YuxiangZhuang]

1. I encounter the problem in pip "requirements-unit1.txt", here is my problem stack:

Downloading gymnasium-0.28.1-py3-none-any.whl (925 kB)
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 925.5/925.5 kB 20.3 MB/s eta 0:00:00
Using cached huggingface_sb3-3.0-py3-none-any.whl (9.7 kB)
Building wheels for collected packages: pygame
error: subprocess-exited-with-error

× python setup.py bdist_wheel did not run successfully.
│ exit code: 1
╰─> See above for output.

note: This error originates from a subprocess, and is likely not a problem with pip.
Building wheel for pygame (setup.py) ... error
ERROR: Failed building wheel for pygame
Running setup.py clean for pygame
Failed to build pygame
ERROR: ERROR: Failed to build installable wheels for some pyproject.toml based projects (pygame)

I found that the

stable-baselines3==2.0.0a5(line 1)

in https://raw.githubusercontent.com/huggingface/deep-rl-class/main/notebooks/unit1/requirements-unit1.txt conflict the pygame needed by gymnasium[box2d](line 3), so i just remove the required version of stable-baselines3 and try pip again in terminal of colab, everything work fine.

2. Environment version v2 for LunarLander is deprecated.

[YuxiangZhuang]

@sambitmukherjee please check this PR, thank you.