docs: add migration note for pendulum removal

[devin-ai-integration]

Added a migration note to document the removal of Pendulum as a transitive dependency in CDK version 6.28.0.

Requested by: Aaron

Link to Devin run: https://app.devin.ai/sessions/41605cb9124946e09b284ca1b28999fa

cc @ian-at-airbyte for documentation review

[devin-ai-integration]

🤖 Devin AI Engineer

Original prompt from Aaron:

@Devin - We removed Pendulum in the latest version of the Python CDK. While this wasn't intended to be a breaking change, I'm finding that many many many Python connectors were using Pendulum directly without declaring their dependency on it. Can you add a note to our CDK migration guide (<https://github.com/airbytehq/airbyte-python-cdk/blob/main/cdk-migrations.md>) that as of this version, if connectors have an undeclared reliance on the transitive pendulum dependency, they will need to declare it going forward. I'll ask you to locate the exact version number and then create the update to the markdown file.

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