Update dart style recommendation for wild cards

[mit-mit]

Dart 3.7 added new support for wild card variables. With that we should probably update this style recommendation?

https://dart.dev/effective-dart/style#prefer-using-_-__-etc-for-unused-callback-parameters

[parlough]

A contributor opened a PR to make updates here and Marya is pondering some other changes as well: #6432 (comment)

[MaryaBelanger]

@mit-mit if you could weigh in on that PR with how you think it should be handled that would be great!

[mit-mit]

I defer to Bob for the style guide, but I think we should consider to document the latest language as the default. We could add a version note for the older style, but perhaps even that isn't needed?

[MaryaBelanger]

Reopening this in case we want to make more changes. Currently, we've just added the note from #6432 (which closed this issue), but we should probably still change the entry all together.

Parker had a good idea to change the title of the section to: "PREFER using wildcards for unused callback parameters", which I think is the best idea too. That would require a little rewriting of the of the text in the section too, and maybe result in removing the new version note.