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 `///`).

add_to_app/multiple_flutters/multiple_flutters_module/pubspec.yaml

@@ -1,10 +1,10 @@
 name: multiple_flutters_module
 description: A module that is embedded in the multiple_flutters_ios and multiple_flutters_android sample code.
-
 version: 1.0.0+1
+resolution: workspace
 
 environment:
-  sdk: ^3.7.0-0
+  sdk: ^3.9.0-0
 
 dependencies:
   flutter: