refactor(shell)!: remove deprecated open API (#3604)

* refactor(shell)!: remove deprecated open API

`open` has been deprecated since 2.1.0 in favor of tauri-plugin-opener.
This removes:

- the `open` command and its `allow-open`/`deny-open` permissions
  (`shell:default` now grants nothing)
- the `plugins > shell > open` configuration; `init()` returns
  `TauriPlugin<R>` again
- `Shell::open`, the `tauri_plugin_shell::open` module and
  `Error::UnknownProgramName`
- the `open` JavaScript function
- the Android and iOS plugins, since `open` was their only command

* chore(examples): drop the now empty shell:default permission set

---------

Co-authored-by: Lucas Nogueira <lucas@crabnebula.dev>

plugins/shell/README.md

@@ -1,14 +1,16 @@
 ![plugin-shell](https://github.com/tauri-apps/plugins-workspace/raw/v2/plugins/shell/banner.png)
 
-Access the system shell. Allows you to spawn child processes and manage files and URLs using their default application.
+Access the system shell. Allows you to spawn child processes.
+
+To open files and URLs with their default application, use [tauri-plugin-opener](../opener).
 
 | Platform | Supported |
 | -------- | --------- |
 | Linux    | ✓         |
 | Windows  | ✓         |
 | macOS    | ✓         |
-| Android  | ✓         |
-| iOS      | ✓         |
+| Android  | x         |
+| iOS      | x         |
 
 ## Install