chore(docs): add deprecation message for languages (aws#21754)

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### 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*

README.md

@@ -28,6 +28,8 @@ The CDK is available in the following languages:
 * .NET ([.NET Core ≥ 3.1](https://dotnet.microsoft.com/download))
 * Go ([Go ≥ 1.16.4](https://golang.org/))
 
+Third-party Language Deprecation: language version is only supported until its EOL (End Of Life) shared by the vendor or community and is subject to change with prior notice.
+
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