Update READMEs for archiving (#7210)

Adds notes to README and CONTRIBUTING pointing to flutter/packages.

Author: stuartmorgan-g

CONTRIBUTING.md

@@ -1,6 +1,10 @@
 # Contributing to Flutter Plugins
 
-[![Build Status](https://api.cirrus-ci.com/github/flutter/plugins.svg)](https://cirrus-ci.com/github/flutter/plugins/main)
+| **ARCHIVED** |
+|--------------|
+| This repository is no longer in use; all source has moved to [flutter/packages](https://github.com/flutter/packages) and future development work will be done there. |
+
+## ARCHIVED CONTENT
 
 _See also: [Flutter's code of conduct](https://github.com/flutter/flutter/blob/master/CODE_OF_CONDUCT.md)_
 

README.md

@@ -1,8 +1,10 @@
 # Flutter plugins
 
-[![Build Status](https://api.cirrus-ci.com/github/flutter/plugins.svg)](https://cirrus-ci.com/github/flutter/plugins/main)
-[![Release Status](https://github.com/flutter/plugins/actions/workflows/release.yml/badge.svg)](https://github.com/flutter/plugins/actions/workflows/release.yml)
-[![OpenSSF Scorecard](https://api.securityscorecards.dev/projects/github.com/flutter/plugins/badge)](https://deps.dev/project/github/flutter%2Fplugins)
+| **ARCHIVED** |
+|--------------|
+| This repository is no longer in use; all source has moved to [flutter/packages](https://github.com/flutter/packages) and future development work will be done there. |
+
+## ARCHIVED CONTENT
 
 This repo is a companion repo to the main [flutter
 repo](https://github.com/flutter/flutter). It contains the source code for