feat(pipelines): CDK Pipelines is now Generally Available (#15667)

Switch the CDK Pipelines API to stable and GA.


----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/pipelines/README.md

@@ -3,13 +3,7 @@
 
 ---
 
-![cdk-constructs: Developer Preview](https://img.shields.io/badge/cdk--constructs-developer--preview-informational.svg?style=for-the-badge)
-
-> The APIs of higher level constructs in this module are in **developer preview** before they
-> become stable. We will only make breaking changes to address unforeseen API issues. Therefore,
-> these APIs are not subject to [Semantic Versioning](https://semver.org/), and breaking changes
-> will be announced in release notes. This means that while you may use them, you may need to
-> update your source code when upgrading to a newer version of this package.
+![cdk-constructs: Stable](https://img.shields.io/badge/cdk--constructs-stable-success.svg?style=for-the-badge)
 
 ---
 

packages/@aws-cdk/pipelines/package.json

@@ -95,8 +95,8 @@
     "node": ">= 10.13.0 <13 || >=13.7.0"
   },
   "license": "Apache-2.0",
-  "stability": "experimental",
-  "maturity": "developer-preview",
+  "stability": "stable",
+  "maturity": "stable",
   "cdk-build": {
     "jest": true,
     "env": {