Move context menus out of experimental (#2134)

Now that the context menu code is on stable, move the sample out of
experimental.

Fixes https://github.com/flutter/samples/issues/2110

## 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 updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

---------

Co-authored-by: Brett Morgan <brett.morgan@gmail.com>

context_menus/.metadata

@@ -0,0 +1,45 @@
+# This file tracks properties of this Flutter project.
+# Used by Flutter tool to assess capabilities and perform upgrades etc.
+#
+# This file should be version controlled.
+
+version:
+  revision: 0df8557c56a182d31fa024eeb08c428ae52edf7f
+  channel: beta
+
+project_type: app
+
+# Tracks metadata for the flutter migrate command
+migration:
+  platforms:
+    - platform: root
+      create_revision: 0df8557c56a182d31fa024eeb08c428ae52edf7f
+      base_revision: 0df8557c56a182d31fa024eeb08c428ae52edf7f
+    - platform: android
+      create_revision: 0df8557c56a182d31fa024eeb08c428ae52edf7f
+      base_revision: 0df8557c56a182d31fa024eeb08c428ae52edf7f
+    - platform: ios
+      create_revision: 0df8557c56a182d31fa024eeb08c428ae52edf7f
+      base_revision: 0df8557c56a182d31fa024eeb08c428ae52edf7f
+    - platform: linux
+      create_revision: 0df8557c56a182d31fa024eeb08c428ae52edf7f
+      base_revision: 0df8557c56a182d31fa024eeb08c428ae52edf7f
+    - platform: macos
+      create_revision: 0df8557c56a182d31fa024eeb08c428ae52edf7f
+      base_revision: 0df8557c56a182d31fa024eeb08c428ae52edf7f
+    - platform: web
+      create_revision: 0df8557c56a182d31fa024eeb08c428ae52edf7f
+      base_revision: 0df8557c56a182d31fa024eeb08c428ae52edf7f
+    - platform: windows
+      create_revision: 0df8557c56a182d31fa024eeb08c428ae52edf7f
+      base_revision: 0df8557c56a182d31fa024eeb08c428ae52edf7f
+
+  # User provided section
+
+  # List of Local paths (relative to this file) that should be
+  # ignored by the migrate tool.
+  #
+  # Files that are not part of the templates will be ignored by default.
+  unmanaged_files:
+    - 'lib/main.dart'
+    - 'ios/Runner.xcodeproj/project.pbxproj'