Updating FAQ to reflect changes in pixi

[faseelmo]

In the latest pixi verson, the [system-requirements] is deprecated and instead virtual packages per platform needs to be described. See here.

As such maybe the last question in the FAQ needs updating to reflect these changes.

It would change from

[system-requirements]
libc = { family = "glibc", version = "2.31" }

to

[workspace]
platforms = [
    { name="robot", platform = "linux-64", glibc = "2.31"}
]

I'm happy to create a PR for this but I was wondering if the existing method in the FAQ should be left in the documentation?