Make 4.18 the default linux system-requirement

[pavelzw]

centos 8 is using 4.18 by default. I would prefer to have this as the default lower bound as there are still a lot of systems that are based on centos 8 where we need to manually change this for every project.
(also i don't think there are many packages that require a higher linux kernel version)

xref https://discord.com/channels/1082332781146800168/1272823440461529140/1272841941515636767

[traversaro]

Today I tried a bunch of pixi projects of the lab on my institute HPC system, and indeed it was quite cumbersome to deal with this. This may be mitigated by #815 or #346, but if there is no advantage in having such a recent linux version (I may be simply ignorant, but I am not aware of any conda-forge package constraining __linux version, to be honest :), see https://github.com/search?q=org%3Aconda-forge+__linux+language%3AYAML&type=code&l=YAML&p=2 ) it would be convenient to just change the default.

[baszalmstra]

I also see no reason not to lower this. The default version came from conda-lock.

Would you be ok with that @ruben-arts ?

[ruben-arts]

The only reason I could see is that if people run pixi update on a project that would silently depend on it (apparently not on conda-forge) They might get older versions than they had before.

But I would agree with you that the benefits outweigh the problems it causes. I'll update it.