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 `///`).

context_menus/lib/default_values_page.dart

@@ -62,7 +62,11 @@ class DefaultValuesPage extends StatelessWidget {
                 'This example simply shows what happens when contextMenuBuilder is given null, a custom value, or omitted altogether.',
               ),
               const SizedBox(height: 40.0),
-              TextField(maxLines: 2, minLines: 2, controller: _controllerNone),
+              TextField(
+                maxLines: 2,
+                minLines: 2,
+                controller: _controllerNone,
+              ),
               TextField(
                 maxLines: 2,
                 minLines: 2,