Update to Draft202012Validator

[a-berish]

Use Draft202012Validator in validator.py

Lines where Draft7 is used

@helrond Yeah that sounds right--from my understanding of the documentation, contains without minContains is the same as setting minContains to 1.

If we're getting rid of minContains, that means we technically could revert back to Draft 7--but we probably want to be on the most recent version of the validator anyway?

Originally posted by @bonniegee in #49 (comment)