[native_assets_cli] Stop reading version

[dcharkes]

Bug: #93

Remove any reads of version in the protocol.

We're moving towards a world where deprecations and breaking changes are per property and documented via the JSON schemas. So we don't use version numbers anymore.

• Change the syntax to make version nullable.
• Still write the last known version (to avoid breaking older hooks and older SDKs with newer SDKs and hooks).
• Remove the version from the semantic API.
• Remove all checks that were checking for versions.
  • Note: this regresses error messages, and this is intended.

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart