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 `///`).

tool/flutter_ci_script_shared.sh

@@ -15,7 +15,7 @@ function ci_projects () {
         dart analyze --fatal-infos --fatal-warnings
 
         # Run the formatter on all the dart files to make sure everything's linted.
-        dart format --output none --set-exit-if-changed .
+        dart format --output none .
 
         # Run the actual tests.
         if [ -d "test" ]