docs: reflect aws-cdk-lib and CLI split in the contribution guide

[rantoniuk]

Remove references to cdk --version check since this doesn't work now after CLI split from aws-cdk.

Closes #34610

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AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: cf77c9a
• Result: SUCCEEDED
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