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_buttons_page.dart

@@ -52,26 +52,27 @@ class CustomButtonsPage extends StatelessWidget {
                 // Build the default buttons, but make them look custom.
                 // Note that in a real project you may want to build
                 // different buttons depending on the platform.
-                children:
-                    editableTextState.contextMenuButtonItems.map((buttonItem) {
-                      return CupertinoButton(
-                        borderRadius: null,
-                        color: const Color(0xffaaaa00),
-                        disabledColor: const Color(0xffaaaaff),
-                        onPressed: buttonItem.onPressed,
-                        padding: const EdgeInsets.all(10.0),
-                        pressedOpacity: 0.7,
-                        child: SizedBox(
-                          width: 200.0,
-                          child: Text(
-                            CupertinoTextSelectionToolbarButton.getButtonLabel(
-                              context,
-                              buttonItem,
-                            ),
-                          ),
+                children: editableTextState.contextMenuButtonItems.map((
+                  buttonItem,
+                ) {
+                  return CupertinoButton(
+                    borderRadius: null,
+                    color: const Color(0xffaaaa00),
+                    disabledColor: const Color(0xffaaaaff),
+                    onPressed: buttonItem.onPressed,
+                    padding: const EdgeInsets.all(10.0),
+                    pressedOpacity: 0.7,
+                    child: SizedBox(
+                      width: 200.0,
+                      child: Text(
+                        CupertinoTextSelectionToolbarButton.getButtonLabel(
+                          context,
+                          buttonItem,
                         ),
-                      );
-                    }).toList(),
+                      ),
+                    ),
+                  );
+                }).toList(),
               );
             },
           ),