Dart 3.9 / Flutter 3.35 [first LLM release] (#2714)

I got carried away with Gemini and basically rewrote CI and the release
process for the new LLM reality. This work was largely completed by
Gemini.

- Bump all SDK versions to the current beta (3.9.0-0)
- Run `flutter channel beta`
- Wrote `ci_script.dart` to replace the bash scripts
- Converted repository to pub workspace #2499 
- Added llm.md and release.md
- Added redirect for deprecated Samples Index

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

form_app/pubspec.yaml

@@ -2,9 +2,10 @@ name: form_app
 description: A sample demonstrating different types of forms and best practices
 publish_to: "none"
 version: 1.0.0+1
+resolution: workspace
 
 environment:
-  sdk: ^3.7.0-0
+  sdk: ^3.9.0-0
 
 dependencies:
   flutter:
@@ -18,7 +19,7 @@ dependencies:
     git:
       url: https://github.com/google/flutter-desktop-embedding.git
       path: plugins/window_size
-  go_router: ^16.1.0
+  go_router: ^16.0.0
 
 dev_dependencies:
   analysis_defaults: