chore: update documentation for .NET requirement and scripts (#34489)

### Reason for this change


Update the .NET requirements in documentation, previously only upgrading the templates.

#32894

### Description of changes

* Update documentation regarding the minimum .NET version

### Describe any new or updated permissions being added


N/A

### Description of how you validated changes


N/A

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

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

Author: berviantoleo

CONTRIBUTING.md

@@ -218,7 +218,7 @@ The following tools need to be installed on your system prior to installing the
 - [Node.js >= 20.x](https://nodejs.org/en/download)
   - We recommend using a version in [Active LTS](https://nodejs.org/en/about/releases/)
 - [Yarn >= 1.19.1, < 2](https://yarnpkg.com/lang/en/docs/install)
-- [.NET SDK >= 6.0.x](https://www.microsoft.com/net/download)
+- [.NET SDK >= 8.0.x](https://www.microsoft.com/net/download)
 - [Python >= 3.8.0, < 4.0](https://www.python.org/downloads/release/python-380/)
 - Either [Docker >= 19.03](https://docs.docker.com/get-docker/), [Finch >= 0.3.0](https://runfinch.com/), or another Docker replacement
   - If using a Docker replacement, the `CDK_DOCKER` environment variable must be set to the replacement command's name (e.g. `export CDK_DOCKER=finch`)

README.md

@@ -24,7 +24,7 @@ The CDK is available in the following languages:
   * We recommend using a version in [Active LTS](https://nodejs.org/en/about/previous-releases)
 * Python ([Python ≥ 3.8](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 ≥ 6.0](https://dotnet.microsoft.com/download))
+* .NET ([.NET ≥ 8.0](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.

scripts/check-build-prerequisites.sh

@@ -121,7 +121,7 @@ else
     fi
 fi
 
-# [.NET == 6.0.x]
+# [.NET >= 6.0.x]
 app="dotnet"
 app_min="6.0.100"
 check_which $app $app_min