Why do model and source yml files always start with `version: 2`?

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.