fix 'new Subject()' and 'new Observable()'

[dynst]

Description:

As pointed out in #7536, the docs for rxjs 7+ have specifically said this since #5307:

https://github.com/ReactiveX/rxjs/blob/c15b37f81ba5f5abea8c872b0189a70b150df4cb/apps/rxjs.dev/content/guide/subject.md#L368

https://github.com/ReactiveX/rxjs/blob/c15b37f81ba5f5abea8c872b0189a70b150df4cb/apps/rxjs.dev/content/guide/subject.md#L377

But the <T = void> was removed for unexplained reasons in #5430 even though the docs were never updated.

BREAKING CHANGE:

Code that depends on new Subject() returning Subject<unknown> will break.

Related issue (if exists):

This also had to fix tests missing type parameters from #3449 #5750 #6216 #6478