fix: revert unnecessary breaking change for resolveType

[n1ru4l]

We can introduce the following type signature for avoiding the breakage of many libraries. In general, I think stuff should first be deprecated in a major release and then finally be removed in the next major release.

export type GraphQLTypeResolver<TSource, TContext> = (
  value: TSource,
  context: TContext,
  info: GraphQLResolveInfo,
  abstractType: GraphQLAbstractType,
) => PromiseOrValue<GraphQLObjectType | string | undefined>;`

[IvanGoncharov]

@n1ru4l Returning string was supported for a long time so I don't see an issue in libraries switching to returning names.

I think stuff should first be deprecated in a major release and then finally be removed in the next major release.

It's done for all the times where the library can't support multiple versions of graphql-js at the same time.
For this particular change, a library can switch to returning a string and support multiple versions of graphql-js at the same time.
But happy to merge if you can provide an example of a library that can't support v15 and v16 in the same release.

[n1ru4l]

@IvanGoncharov I double-checked v15.5.1 and you are right, I was not aware of this! This should not be necessary then.