chore: update check-build-prerequisites.sh to include .NET 9

[ashishdhingra]

Issue # (if applicable)

Closes #32282.

Reason for this change

Add support for .NET 9 if it is the only SDK installed on user's machine.

Description of changes

Updates check-build-prerequisites.sh to include .NET 9.

Description of how you validated changes

Ran expression locally echo $app_v | grep -c -E "[6789]\.[0-9]+\.[0-9]+.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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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

[aws-cdk-automation]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.46%. Comparing base (7cc5f30) to head (c22f921).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #32298   +/-   ##
=======================================
  Coverage   77.46%   77.46%           
=======================================
  Files         105      105           
  Lines        7168     7168           
  Branches     1314     1314           
=======================================
  Hits         5553     5553           
  Misses       1433     1433           
  Partials      182      182           

Flag	Coverage Δ
suite.unit	77.46% <ø> (ø)

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Components	Coverage Δ
packages/aws-cdk	77.46% <ø> (ø)

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.

[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: c22f921
• Result: SUCCEEDED
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