Update license headers

[ericcornelissen]

Summary

The license headers in source files currently state

ghasum/internal/cache/cache.go

Line 1 in 82d6993

// Copyright 2024 Eric Cornelissen

this should probably be changed now that the project has been moved to the chains-project organization to reflect the fact that others will likely contribute too.

I'm actually not sure what to put there in this case. I've seen something like "Copyright 2024 ghasum contributors" in the past but also recall reading that's probably not legally meaningful.