Link to wiki page about updating dependencies in each `pubspec.yaml` file (#140826)

Added a minimal wiki page about how to use the `update-packages` tool: https://github.com/flutter/flutter/wiki/Updating-dependencies-in-Flutter

This PR adds a link to that wiki page at the top of each pubspec.yaml file

packages/flutter/pubspec.yaml

@@ -7,6 +7,9 @@ environment:
 
 dependencies:
   # To update these, use "flutter update-packages --force-upgrade".
+  #
+  # For detailed instructions, refer to:
+  # https://github.com/flutter/flutter/wiki/Updating-dependencies-in-Flutter
   characters: 1.3.0
   collection: 1.18.0
   material_color_utilities: 0.8.0

packages/flutter/test_private/pubspec.yaml

@@ -6,6 +6,9 @@ environment:
 
 dependencies:
   # To update these, use "flutter update-packages --force-upgrade".
+  #
+  # For detailed instructions, refer to:
+  # https://github.com/flutter/flutter/wiki/Updating-dependencies-in-Flutter
   meta: 1.11.0
   path: 1.9.0
   process: 5.0.1

packages/flutter/test_private/test/pubspec.yaml

@@ -5,6 +5,9 @@ environment:
 
 dependencies:
   # To update these, use "flutter update-packages --force-upgrade".
+  #
+  # For detailed instructions, refer to:
+  # https://github.com/flutter/flutter/wiki/Updating-dependencies-in-Flutter
   flutter:
     sdk: flutter
   flutter_test:

packages/flutter_goldens/pubspec.yaml

@@ -5,6 +5,9 @@ environment:
 
 dependencies:
   # To update these, use "flutter update-packages --force-upgrade".
+  #
+  # For detailed instructions, refer to:
+  # https://github.com/flutter/flutter/wiki/Updating-dependencies-in-Flutter
   flutter:
     sdk: flutter
   flutter_test:

packages/flutter_localizations/pubspec.yaml

@@ -5,6 +5,9 @@ environment:
 
 dependencies:
   # To update these, use "flutter update-packages --force-upgrade".
+  #
+  # For detailed instructions, refer to:
+  # https://github.com/flutter/flutter/wiki/Updating-dependencies-in-Flutter
 
   flutter:
     sdk: flutter

packages/flutter_test/pubspec.yaml

@@ -5,6 +5,9 @@ environment:
 
 dependencies:
   # To update these, use "flutter update-packages --force-upgrade".
+  #
+  # For detailed instructions, refer to:
+  # https://github.com/flutter/flutter/wiki/Updating-dependencies-in-Flutter
 
   flutter:
     sdk: flutter

packages/flutter_tools/pubspec.yaml

@@ -7,6 +7,9 @@ environment:
 
 dependencies:
   # To update these, use "flutter update-packages --force-upgrade".
+  #
+  # For detailed instructions, refer to:
+  # https://github.com/flutter/flutter/wiki/Updating-dependencies-in-Flutter
   archive: 3.3.2
   args: 2.4.2
   browser_launcher: 1.1.1