Deprecate setting schema paths to primitives (like 'foo: false')

[vkarpov15]

The below schema sets 'foo' to a mixed type:

new Schema({
  name: String,
  foo: false,
});

This is a potentially dangerous gotcha. We'll still allow _id: false as a special case, because that's become a pervasive meme in Mongoose projects and because we already handle it with this commit (re: #7480). But I think for 6.0 we should throw an error in the above case.

Relevant issues: #7512 #7516 #7480 #7524