[10.x] Fix typehint for EventServiceProvider::$observers property

[bradietilley]

Issue:

New typehint was added d800f9e which is too strict (only supports array of strings).

My syntax is valid (unless string-to-string mapping is now deprecated in which case whoops I missed the memo). AFAICT it's still valid in HasEvents:

---

Also, while you can't instantiate objects directly in the property declaration, it's still important to define a typehint that supports objects, otherwise the following valid logic would appear invalid in static analysis:

// EventServiceProvider
$model = \App\Models\User::class;
$object = new \App\Observers\UserObserver();

$this->observers[$model] = $object;

I mean, I never use objects in this context, but it's technically valid so it should be typehinted as valid too.

[cosmastech]

#46942 (comment)

I had noted that this should be a class-string and not just a string for the docblock type hint too. Maybe @nunomaduro can chime in on that, I'm not sure how much Laravel team uses class-string but I personally love the precision 😄

[bradietilley]

@cosmastech Yeah I was itching to do that too, instead I just emulated what was done with the $listen property. Keen for class-string if that's everyone else's preference 👍