Remove support for `pixi-version` argument

[VeckoTheGecko]

Can be handled via the requires-pixi option in the pixi manifest instead https://pixi.prefix.dev/dev/reference/pixi_manifest/#requires-pixi-optional

[VeckoTheGecko]

Thoughts on this @keewis ? Do you see value in being able to exactly specify requires-pixi ? xref pydata/xarray#11154

[keewis]

it's true that these do similar things, but I would consider requires-pixi to be closer to the version ranges in pixi.toml's dependencies table, while pixi-version does exact pins and would be equivalent to a lock file.

If the above makes sense, I'd say it makes sense to have both (just like setup-pixi has both).

[VeckoTheGecko]

Cool! I was unsure for that reason, thanks for the input :))