update repo references from the move

[devoncarew]

• update repo references from the move (dart-lang/ffigen => dart-lang/native)

There are several additional references to dart-lang/ffigen (in docs, ....); it's worth a pass through the repo to update them.

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[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:ffigen	10.0.0-dev.0	ready to publish	ffigen-v10.0.0-dev.0
package:native_toolchain_c	0.3.2	already published at pub.dev
package:native_assets_builder	0.3.0	already published at pub.dev
package:native_assets_cli	0.3.2	already published at pub.dev

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