Update Kubeflow Pipelines and Apache Airflow example connector documentation

[ptitzler]

Follow-up to https://github.com/elyra-ai/elyra/pull/3141

• Mark the Apache Airflow example component connector as no longer maintained
• Add statement that Apache Airflow example component connector does not support Elyra v4 and above
• Mark the Kubeflow Pipelines example component connector as no longer maintained
• Add statement that Kubeflow Pipelines example component connector does not support Elyra v4 and above

What changes were proposed in this pull request?

See above

How was this pull request tested?

Reviewed updated documentation.

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