deprecate avoid_returning_null under null safety

[kevmoo]

lot's of noise!

see #58100

[pq]

/fyi @munificent @bwilkerson

[bwilkerson]

@kevmoo Could you be more specific about the kind(s) of noise that you're seeing? I'd like to better understand the problem in order to understand the solution.

[kevmoo]

Before null-safety, you don't want giveMeInt() to return null, because it breaks 5 + giveMeInt().

With any package opting into null-safety, you now get the right static hints.

I'd say we say this lint is obsolete/deprecated for null-safe packages

[bwilkerson]

Agreed. Just wanted to know if there was more to it than that.

[pq]

Closing in favor of #58394. (TL;DR: yes, let's!)