Fix: simplify MergeType constraints

Fixes 16 errors in models.d.ts on Typescript 4.8 which is currently in
beta. Typescript 4.8 forbids assignment of an unconstrained type
parameter to one that disallows `null` or `undefined`:
microsoft/TypeScript#49489

The alternative fix is to constrain *lots* of types in models.d.ts to
`Record<string | number, any>` -- that is, to forbid `null | undefined`,
but I'm not sure it's worth the effort.

types/utility.d.ts

@@ -10,6 +10,6 @@ declare module 'mongoose' {
       ? T
       : Omit<T, keyof U> & U;
 
-  type MergeType<A extends Record<number | string, any>, B extends Record<number | string, any>> = Omit<A, keyof B> & B;
+  type MergeType<A, B> = Omit<A, keyof B> & B;
 
 }