CONTRIBUTING.md should say that Python 3.8 is required

### What component of Red (cog, command, API) would you like to see improvements on?

CONTRIBUTING.md

### Describe the enhancement you're suggesting.

Currently CONTRIBUTING.md says that [3.8 or greater](https://github.com/Cog-Creators/Red-DiscordBot/blob/dbd71db6a8392e6fe9b9a8a33501c2595e4f7bac/CONTRIBUTING.md?plain=1#L55) is needed, but when running `make newenv` with Python 3.12 installed, this error appears
```sh
python3.8 -m venv --clear .venv
make: python3.8: No such file or directory
make: *** [newenv] Error 1
```
apparently the CONTRIBUTING.md should be 3.8, not 3.8 or greater: [convo](https://discord.com/channels/133049272517001216/160386989819035648/1290632459217539114)

### Anything else?

_No response_