chore: remove node 13 incompatibility warnings (aws#18284)

Because the required minimum version is 14+, warning about specific
versions of NodeJS 13 seems redundant (as they are likely incompatible
anyway).

Relates to aws#18144 

----

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

CONTRIBUTING.md

@@ -54,7 +54,6 @@ The following tools need to be installed on your system prior to installing the
 
 - [Node.js >= 14.15.0](https://nodejs.org/download/release/latest-v14.x/)
   - We recommend using a version in [Active LTS](https://nodejs.org/en/about/releases/)
-  - ⚠️ versions `13.0.0` to `13.6.0` are not supported due to compatibility issues with our dependencies.
 - [Yarn >= 1.19.1, < 2](https://yarnpkg.com/lang/en/docs/install)
 - [.NET Core SDK 3.1.x](https://www.microsoft.com/net/download)
 - [Python >= 3.6.5, < 4.0](https://www.python.org/downloads/release/python-365/)

README.md

@@ -21,7 +21,6 @@ The CDK is available in the following languages:
 
 * JavaScript, TypeScript ([Node.js ≥ 14.15.0](https://nodejs.org/download/release/latest-v14.x/))
   - We recommend using a version in [Active LTS](https://nodejs.org/en/about/releases/)
-  - ⚠️ versions `13.0.0` to `13.6.0` are not supported due to compatibility issues with our dependencies.
 * Python ([Python ≥ 3.6](https://www.python.org/downloads/))
 * Java ([Java ≥ 8](https://www.oracle.com/technetwork/java/javase/downloads/index.html) and [Maven ≥ 3.5.4](https://maven.apache.org/download.cgi))
 * .NET ([.NET Core ≥ 3.1](https://dotnet.microsoft.com/download))
@@ -78,7 +77,6 @@ For a detailed walkthrough, see the [tutorial](https://docs.aws.amazon.com/cdk/l
 
 ### At a glance
 Install or update the [AWS CDK CLI] from npm (requires [Node.js ≥ 14.15.0](https://nodejs.org/download/release/latest-v14.x/)). We recommend using a version in [Active LTS](https://nodejs.org/en/about/releases/)
-⚠️ versions `13.0.0` to `13.6.0` are not supported due to compatibility issues with our dependencies.
 
 ```console
 $ npm i -g aws-cdk