Previously, `removeInaccessibleElements` would throw an error the first time it encountered an inaccessible mismatch for types & fields.

With this change, the code will continue to parse the schema and report an `AggregateError` that contains all the mismatches.

`ErrGraphQLValidationFailed` supports multiple errors and node12 & node14 don't support es2019 (which is when `AggregateError` was introduced). So rather than monkeying with node versions and polyfills, we'll just use what's already available.