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/custom_menu_page.dart

@@ -47,11 +47,10 @@ class CustomMenuPage extends StatelessWidget {
             contextMenuBuilder: (context, editableTextState) {
               return _MyContextMenu(
                 anchor: editableTextState.contextMenuAnchors.primaryAnchor,
-                children:
-                    AdaptiveTextSelectionToolbar.getAdaptiveButtons(
-                      context,
-                      editableTextState.contextMenuButtonItems,
-                    ).toList(),
+                children: AdaptiveTextSelectionToolbar.getAdaptiveButtons(
+                  context,
+                  editableTextState.contextMenuButtonItems,
+                ).toList(),
               );
             },
           ),