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 `///`).

date_planner/lib/symbol_editor.dart

@@ -39,11 +39,8 @@ class _SymbolEditorState extends State<SymbolEditor> {
                   CupertinoButton(
                     padding: EdgeInsets.zero,
                     child: const Text('Done'),
-                    onPressed:
-                        () => Navigator.pop(context, (
-                          _currentIcon,
-                          _currentColor,
-                        )),
+                    onPressed: () =>
+                        Navigator.pop(context, (_currentIcon, _currentColor)),
                   ),
                 ],
               ),