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

[dynst]

Description:

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

rxjs/apps/rxjs.dev/content/guide/subject.md

Line 368 in c15b37f

const subject = new Subject(); // Shorthand for Subject<void>

rxjs/apps/rxjs.dev/content/guide/subject.md

Line 377 in c15b37f

<span class="informal">Before version 7, the default type of Subject values was `any`. `Subject<any>` disables type checking of the emitted values, whereas `Subject<void>` prevents accidental access to the emitted value. If you want the old behavior, then replace `Subject` with `Subject<any>`.</span>

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