chore: remove experimental badge from aws-cdk-lib README (#17827)

The package was marked as stable, but this bit in the README was missed.

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packages/aws-cdk-lib/README.md

@@ -1,7 +1,5 @@
 # AWS Cloud Development Kit Library
 
-[![experimental](http://badges.github.io/stability-badges/dist/experimental.svg)](http://github.com/badges/stability-badges)
-
 The AWS CDK construct library provides APIs to define your CDK application and add
 CDK constructs to the application.