Add use_null_aware_elements to recommended lint set

[Reprevise]

I was surprised to find that when I upgraded Dart, I wasn't getting any lints to use the new null aware elements feature, then I realized I had to turn on the lint myself even after upgrading the lints package. I think this would be good to add to at least the recommended lint set.

and lints that enforce writing Dart using a single, idiomatic style and format.

I think this lint aligns with the goal of the recommended set, and encourages more people to use the new feature.

There is an open false negative issue about this lint so that should be fixed before this is ever landed: dart-lang/sdk#61122

[natebosch]

I'm in favor of putting this in recommended.