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.

Resource yml files do not currently require this config. We only support version: 2 if it's specified. Although we do not expect to update yml files to version: 3 soon, having this config will make it easier for us to introduce new structures in the future