Once upon a time, the structure of these
.yml files was very different (s/o to anyone who was using dbt back then!). Adding
version: 2 allowed us to make this structure more extensible.
Currently, Version 2 is the only supported version for these files. We kept
version: around as a required key so that in the future, if we need to introduce a new structure for these files, we'll be able to do this more easily.