upgrade chebai to the latest lightning version

[sfluegel05]

As of now, chebai is only compatible with lightning versions up to 2.1, the most recent version (as of 08/24) is 2.4.
The problem with upgrading to 2.4 seems to be a change in the hyperparameter logging: #39 (comment)

For now, I have added lightning<=2.1 as a requirement (see #40). At some point, we should fix the problem and upgrade to the newest lightning version.

[aditya0by0]

Before v2.2.0: if no seed is specified, set random seed
After v2.2.0: if no seed is specified, set seed to 0

#12 (comment)

I wanted to highlight/recall that downgrading the version to 2.1 has the effects mentioned above in the comment.