Both the 6-22 and 6-24/latest channels are C++17, being the latest standard GCC 9 claims to be stable with, new releases will probably remain that way until the snap rebases on Ubuntu 22.04.
I’ll add that information to the store listing, but CERN has a service account that has write access to the snap and can change its description, so if you ever wanted to adjust something yourselves then feel free to use it.