Update samples repo to Swift 6

# Background

Swift 6 language mode enables strict concurrency checking. Code changes may be necessary to upgrade a package to the Swift 6 language mode.

A Swift 6 app can depend on Swift 5 packages and vice-versa. The Swift 6 compiler supports [both Swift 6 and Swift 5 language modes](https://www.swift.org/migration/documentation/swift-6-concurrency-migration-guide/swift6mode/).

WWDC session: https://www.youtube.com/watch?v=75-c6jSE8kU
Migration docs: https://www.swift.org/migration/documentation/swift-6-concurrency-migration-guide/swift6mode/

See also:
* https://github.com/flutter/flutter/issues/150388
* https://github.com/flutter/website/issues/10820

# Work

Update the `flutter/samples` repo to use Swift 6:

https://github.com/search?q=repo%3Aflutter%2Fsamples+path%3A.swift+NOT+path%3AGeneratedPluginRegistrant.swift&type=code