fix(query): revert breaking changes introduced by #12797

[lpizzinidev]

closes #12930

Summary
As a result of this conversation, #12797 introduced breaking changes.
This PR revert them (as well as the fix of #12826) to the previous behavior.

Update queries now do not remove fields set to undefined.

[hasezoey]

LGTM

will the original change be re-introduced for 7.0 then?
also what about #12956, should it be merged first or dropped for 6.x?

[vkarpov15]

I don't think we should re-introduce this change in 7.0. I think the original change was generally incorrect, we do not want to convert undefined keys to $unset. That would be potentially risky for anyone that's passing user specified data as updates to Mongoose, assuming that $set removes undefined keys. And I don't think the benefit is quite there. We can consider adding a utility to convert undefined keys to $unset, but I don't think that should be the default.