docs(aws-lambda): mark NODEJS_12_X as deprecated (#22093)

See https://aws.amazon.com/de/blogs/developer/announcing-the-end-of-support-for-node-js-12-x-in-the-aws-sdk-for-javascript-v3/

----

* [x] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

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

Author: coderbyheart

packages/@aws-cdk/aws-lambda/lib/runtime.ts

@@ -72,6 +72,7 @@ export class Runtime {
 
   /**
    * The NodeJS 12.x runtime (nodejs12.x)
+   * @deprecated Legacy runtime no longer supported by AWS Lambda. Migrate to the latest NodeJS runtime.
    */
   public static readonly NODEJS_12_X = new Runtime('nodejs12.x', RuntimeFamily.NODEJS, { supportsInlineCode: true });