chore(lambda): declare dotnetcore3.1 deprecated (aws#22969)

----
source: https://docs.aws.amazon.com/lambda/latest/dg/lambda-runtimes.html
chore(lambda): declare dotnetcore3.1 deprecated

### All Submissions:

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

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

*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

@@ -161,24 +161,25 @@ export class Runtime {
 
   /**
    * The .NET Core 1.0 runtime (dotnetcore1.0)
-   * Legacy runtime no longer supported by AWS Lambda. Migrate to the latest .NET Core runtime.
+   * @deprecated Legacy runtime no longer supported by AWS Lambda. Migrate to the latest .NET Core runtime.
    */
   public static readonly DOTNET_CORE_1 = new Runtime('dotnetcore1.0', RuntimeFamily.DOTNET_CORE);
 
   /**
    * The .NET Core 2.0 runtime (dotnetcore2.0)
-   * Legacy runtime no longer supported by AWS Lambda. Migrate to the latest .NET Core runtime.
+   * @deprecated Legacy runtime no longer supported by AWS Lambda. Migrate to the latest .NET Core runtime.
    */
   public static readonly DOTNET_CORE_2 = new Runtime('dotnetcore2.0', RuntimeFamily.DOTNET_CORE);
 
   /**
    * The .NET Core 2.1 runtime (dotnetcore2.1)
-   * Legacy runtime no longer supported by AWS Lambda. Migrate to the latest .NET Core runtime.
+   * @deprecated Legacy runtime no longer supported by AWS Lambda. Migrate to the latest .NET Core runtime.
    */
   public static readonly DOTNET_CORE_2_1 = new Runtime('dotnetcore2.1', RuntimeFamily.DOTNET_CORE);
 
   /**
    * The .NET Core 3.1 runtime (dotnetcore3.1)
+   * @deprecated Legacy runtime no longer supported by AWS Lambda. Migrate to the latest .NET Core runtime.
    */
   public static readonly DOTNET_CORE_3_1 = new Runtime('dotnetcore3.1', RuntimeFamily.DOTNET_CORE);
 
@@ -189,7 +190,7 @@ export class Runtime {
 
   /**
    * The Ruby 2.5 runtime (ruby2.5)
-   * Legacy runtime no longer supported by AWS Lambda. Migrate to the latest Ruby runtime.
+   * @deprecated Legacy runtime no longer supported by AWS Lambda. Migrate to the latest Ruby runtime.
    */
   public static readonly RUBY_2_5 = new Runtime('ruby2.5', RuntimeFamily.RUBY);