feat(auth): update Pigeon version to 19

[Lyokone]

Description

Aligning all constraints to Web constraints. As discussed before, it was already the case with the previous version but not reflected in the front facing package.

Also updated the Flutter SDK constraints, (Dart 3.2 meant Flutter 3.16 already)

• (iOS) Booleans were held in a NSNumber which is not the case anymore (nice but annoying since no linting errors)
• EventChannels decoding system has changed
• Lot of signature/naming changed

Related Issues

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Checklist

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• I updated/added relevant documentation (doc comments with ///).
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Breaking Change

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• No, this is not a breaking change.