chore: upgrade dependencies; remove freezed

[btrautmann]

📰 Summary of changes

What is the new functionality added in this PR?

This PR does the following (and includes ⚠️ Breaking changes ⚠️ ):

• Updates dependencies, including the minimum required dart version
• Drops the freezed and freezed_annotation dependencies which were previously used only to give us sealed-like classes. Since sealed class became a thing after this library was designed, we can drop that and used sealed directly. Note that the public API/constructors of these types (NetworkRequestBody and SturdyHttpEvent) has changed and will require adjustments (which should be doable via Find All + Replace).
• Changes the conditions under which DecodingErrors are emitted. See the comment below.

🧪 Testing done

Unit test coverage feels sufficient here.

[samandmoore]

domainlgtm

just a few questions about stuff that we might not need anymore as well

[samandmoore]

do we need to update the readme too? any examples?

[btrautmann]

Note: This is a behavior change. Previously, we'd only emit a DecodingError event if the Exception was a CheckedFromJsonException. That felt a bit smelly to me (it relies on the consumer's decoding implementation. I.e. if they used dart_mappable, a MapperException is thrown instead, and this check wouldn't ever emit). Now, we emit this event on every Exception thrown during deserialization. This encourages two things:

• Keeping onResponse logic focused on just deserialization to minimize the chance of an Exception being thrown that's not related to deserialization
• Allowing consumers themselves to decide if they "react" to a DecodingError (they can check if the passed Exception was indeed a variant they care about).

[samandmoore]

domainlgtm

the exception event emitting behavior change makes sense to me too.

nice clean up!

[btrautmann]

/no-platform