[flutter_adaptive_scaffold] Surface `useLegacyColorScheme` property in `AdaptiveScaffold`

[a-wallen]

Surfaces the useLegacyColorScheme property from BottomNavigationBar.

Fixes https://github.com/flutter/flutter/issues/116903

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• [x] I signed the CLA.
• [x] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• [x] I listed at least one issue that this PR fixes in the description above.
• [x] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• [x] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• [x] I updated/added relevant documentation (doc comments with ///).
• [ ] I added new tests to check the change I am making, or this PR is test-exempt.
• [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[stuartmorgan]

Closing as this has been draft for a while; please feel free to re-open if this is something you plan to revisit.

[a-wallen]

@stuartmorgan no problem. FWIW this PR can land once useLegacyColorScheme rolls into stable, but since that property hasn't rolled on that branch I marked the PR as a draft. I'm not sure if that changes your decision about closing that PR.

[stuartmorgan]

Sure, if it's good to go other than that, leaving it open makes sense. Thanks!

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[gspencergoog]

Oh, I guess it's not defined in 3.3.10 (it is in the stable 3.6.0 branch in my Flutter, though).

I guess we'll wait until the legacy version catches up.

[a-wallen]

SGTM. I guess I'll be seeing you again when that happens @gspencergoog.

[stuartmorgan]

Syncing with main now that everything has rolled forward for the 3.10 release.

[stuartmorgan]

I pinged @Hixie in Discord about the CLA problem.

[stuartmorgan]

The CLA issue is fixed, but we'll need to update this to resolve the conflicts. CHANGELOG is the usual trivial change, but there's a code conflict too.

[stuartmorgan]

@gspencergoog Do you have time to fix the conflicts here, or should I add that to my list?

[gspencergoog]

I could fix the conflicts, but NavigationBar doesn't even have useLegacyColorScheme, so I'm thinking that we could just close this PR, since we're not using BottomNavigationBar anymore.