Upgrade verilator

[nasahlpa]

In the future, it might be worth to upgrade Verilator to a more recent version. Especially when dealing with linting errors, a more recent version could get rid of some hacks to avoid certain linting errors (e.g., #2129).

[nasahlpa]

The main reason why I would suggest upgrading to a more recent Verilator version (e.g. >=5) is that I am running in these Warning-IMPERFECTSCH from the Verilator linting step. It seems that this is a historical feature which is not used anymore in Verilator >=5. Does anybody (maybe @GregAC or @rswarbrick) know, why we enabled this check with the -Wwarn-IMPERFECTSCH?
Instead of upgrading to a more recent version of Verilator, we also could disable this flag.