[infra] Use Pub Workspaces

[Levi-Lesches]

Closes #1223

• Update all packages to require Dart 3.6 or higher
• Use Pub workspaces for all packages

TODO:

• [x] Finish remaining packages
• [x] Verify that native_assets_builder/test_data/* still works with dart pub get
• [x] Verify that native_assets_cli/example/* still works with dart pub get
• [x] Verify that dart analyze passes at the workspace level`
• [x] Bump CI to use Dart 3.6 for all packages (stable/beta, migrated dev -> stable)
• [x] Consolidate analysis_options.yaml into workspace
• [ ] Verify tests still run (waiting for CI)
• [x] Update changelogs and pubspec versions (Is this needed?)

Leaving these packages unmigrated:

• jnigen/android_test_runner
• native_assets_builder/test_data/native_add_version_skew

Blocked by:

• #1905
• package:firehose needs to use package:dart_apitool: ^0.20.2

---

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