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/event_detail.dart

@@ -55,9 +55,8 @@ class _EventDetailState extends State<EventDetail> {
                           Navigator.of(context)
                               .push(
                                 CupertinoPageRoute<(IconData, ColorOptions)?>(
-                                  builder:
-                                      (_) =>
-                                          SymbolEditor(event.icon, event.color),
+                                  builder: (_) =>
+                                      SymbolEditor(event.icon, event.color),
                                 ),
                               )
                               .then(((IconData, ColorOptions)? data) {