feat(lambda): dotnet6 runtime (aws#19144)

fix aws/aws-lambda-dotnet#1088, fix aws#19148
https://aws.amazon.com/about-aws/whats-new/2022/02/aws-lambda-adds-support-net6/
https://aws.amazon.com/blogs/compute/introducing-the-net-6-runtime-for-aws-lambda/
https://docs.aws.amazon.com/lambda/latest/dg/lambda-runtimes.html


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

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

@@ -137,6 +137,11 @@ export class Runtime {
     supportsCodeGuruProfiling: true,
   });
 
+  /**
+   * The .NET 6 runtime (dotnet6)
+   */
+  public static readonly DOTNET_6 = new Runtime('dotnet6', RuntimeFamily.DOTNET_CORE);
+
   /**
    * The .NET Core 1.0 runtime (dotnetcore1.0)
    * Legacy runtime no longer supported by AWS Lambda.