Add a way to not show the same warning each time a snap run

[maxf]

Hello, using snap 2.49.2 on Debian sid here. I think I understand there's some WIP regarding a cgroup v2 thing, and for now we're stuck with the warning "WARNING: cgroup v2 is not fully supported yet, proceeding with partial confinement" each time a snap run.
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The issue is I use the `lxc exec` command a lot in scripts and the result is my output is literally spammed with this warning.

‎I am now well aware of it, having read it hundreds of time in a short period of time ;). Furthermore it makes reading the useful output very hard.
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Trying to set system.kernel.printk.console-loglevel from 0 to 7 didn't change anything.

I was told on IRC that the above setting couldn't help me (we're not dealing with a kernel level warning) and that there's currently no way to suppress the warning. The same person suggested I filled a bug asking for an environment flag we could use to disable the warning.

‎Another person humoristically suggested the flag `SNAPD_SHUT_UP=true` ;)

Regards

[Stefano Rivera (stefanor)]

Please consider bumping the severity, this doesn't just cause annoyance, but also breaks things that expect nothing on stderr. See https://bugs.debian.org/996036

[Ian Johnson (anonymouse67)]

This output will be gone in snapd 2.53 I think since we have finished cgroupsv2 support, see https://github.com/snapcore/snapd/pull/10776 which will be included in snapd 2.53.