Set log_level to critical with --json option

[Hind-M]

Fix #2446
I don't know the reason behind setting log_level to off when dealing with --json but I guess it shouldn't be the case.
Let's see if the CI tests fail...

[jonashaag]

Is it because the log output is incorrectly sent to stdout and this was a workaround? JSON output should go to stdout and errors to stderr

[Hind-M]

Yeah, most certainly...

[jonashaag]

Are we now sending logs and JSON output to stdout?

[Hind-M]

We are planning to completely rework the logging system of mamba by the end of this year, but since it's still some way off, bugs should be fixed meanwhile.
In this fix, critical errors will be shown in the case of json as well. But I believe we are not changing anything else.
(If there are any critical errors, I don't think json output would be relevant anyway...)
Note that when fixing #2458, I noticed that json output is sent to stdout because we use conda which is again showing the inconsistencies when handling all this logging...