Impl `StyleEditionDefault` trait for all configs

[ytmimi]

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This PR is a follow up to https://github.com/rust-lang/rustfmt/pull/5933, ~~but currently includes all the changes from that PR since it hasn't been merged yet~~.

This PR makes internal changes to define each configuration in terms of the StyleEditionDefault trait, and hard codes StyleEdition::Edition2015 anywhere the new style_edition is needed.

Note users are still unable to configure style edition.

r? @calebcartwright

Edit: https://github.com/rust-lang/rustfmt/pull/5933 was merged and I rebased this PR.

[calebcartwright]

@ytmimi do you think this would be better to merge before or after the next sync? (no promises but I'm going to try to do one today)

[ytmimi]

@calebcartwright If we're going to do a sync we should update the changelog. I've started that on #6004, but there are a few other fixes that I merged that don't have entries yet.

[ytmimi]

That said, we can probably wait to merge this until after the sync

[ytmimi]

Diff check job passed ✅