add AIRFLOW_VAR_ prefix to environment variables in data-platform blueprints

[Tutuchan]

As discussed in #1649, this PR adds the AIRFLOW_VAR_ prefix to all environment variables in the data-platform-foundations and data-platform-minimal blueprints, to enable them being used as Airflow Variables.

I also updated the example DAGs based on this change + took the liberty to remove unused imports.

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Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass

I add one test failing locally -- tests/modules/cloud_function_v2/examples/iam.yaml but I don't see how it's related to this change.

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[lcaggio]

Thanks for the PR!

[Tutuchan]

Thanks for the approval @lcaggio, I'm waiting for an answer internally about the CLA, I'll get back to the PR when it has moved forward.

[Tutuchan]

Just FYI, CLA has been approved internally, we are waiting for the Google review which should take a few days.

[ludoo]

Fixes #1649