Update docs to refer to the proper Debian version

[IkerGarcia]

Hi,

I was following the guide but didn't work properly under Debian WSL stretch. Asked in discord, upgrade to buster and now works ok. It should be pointed out in the docs.

[Daft-Freak]

Quoting myself:

Hmm, should probably document required versions of stuff. Linux/WSL docs pretty much assume "recent-ish Debian-based"

Some required versions:

• gcc-arm-none-eabi 7.x
• CMake 3.9
• Python 3.6

So... Ubuntu 20.04 (or even 18.04 with a newer toolchain), but Debian buster (not even stretch + backports)

[Daft-Freak]

Adding this here so I don't forget: gcc-arm-none-eabi 10.x doesn't work... need to debug that.

[Daft-Freak]

Made some improvements in #660

[ahnlak]

While this is pretty ancient history, I'm guessing that since 727e057 we should be able to close this?

[Daft-Freak]

Hmm, might need to change the debian requirement to bullseye. (Updated the GCC requirement in 96b4618... and buster does not have 8.x, oops.)

This is entirely because I don't have a buster install to check though.