resolvers skip asynchronous validators

[m-lyon]

The validateDoc function used in many resolvers, e.g. updateById, createOne, found here will call doc.validateSync for mongoose >= v7. validateSync does not run asynchronous validators, and instead skips them. This leads to validation being silently skipped within this package.

Since validateDoc is an asynchronous function, and awaits for validations in mongoose < v7, it seems that there is no good reason to not await for validation in the >= v7 case as well.

Happy to make a pull request if the maintainers agree.