cleanup: Upgrade Composer samples to Airflow 2.0

[leahecole]

Description

This PR follows best practices for upgrading DAGs from Airflow 1 to Airflow 2. Changes included:

• Adding an additional conftest.py to the blogs folder. For now, that DAG will stay at using Airflow 1.0, and we need to initialize a compatible DB for those tests separate from the workflows DB.
• Use of the newly released unit test util in workflows/requirements-test.txt
• Imports changed from contrib to providers
• Removal of samples that showcased running Python 2 code
• Referring to the officially supported Airflow constraints file instead of the piecemeal one I had
• Making the appropriate changes on anything previously throwing a Deprecation Warning (i.e. bash_operator -> bash)

If you would like access to my Composer environment to see the DAGs deployed, please let me know. Otherwise, here is a screenshot showing successful dag runs across the board

Fixes #5171
DataflowTemplateOperator tutorial update in CL/371792521 - removed instructions for setting gce_region Airflow variable (@sofisl )

CHANGE NOT ADDRESSED - I need to look further at the DAG example we have triggered by GCF. That will be addressed in an upcoming PR.

Note: It's a good idea to open an issue first for discussion.

Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• README is updated to include all relevant information
• Tests pass: nox -s py-3.6 (see Test Environment Setup)
• Lint pass: nox -s lint (see Test Environment Setup)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• Please merge this PR for me once it is approved.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample

[snippet-bot]

You are about to delete the following sample browser pages.

• composer/workflows/bashoperator_python2.py:15, tag composer_bashoperator_python2(usage: page 1).
• composer/workflows/pythonvirtualenvoperator_python2.py:15, tag composer_pythonvirtualenvoperator_python2(usage: page 1).

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Here is the summary of changes.

You are about to add 1 region tag.

• composer/workflows/bq_notify.py:42, tag composer_bigquery

You are about to delete 3 region tags.

• composer/workflows/bashoperator_python2.py:15, tag composer_bashoperator_python2
• composer/workflows/bq_notify.py:37, tag composer_bigquery
• composer/workflows/pythonvirtualenvoperator_python2.py:15, tag composer_pythonvirtualenvoperator_python2

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[leahecole]

YAY #5806 made the tests pass!! This is only red because I'm bad at remembering to run lint locally!!!!!!!!!!!!!

[sofisl]

Just looked at the dataflow/dataproc tutorials, LGTM!

[rachael-ds]

LGTM!

[leahecole]

omg I have no idea how I made this mess with adding these requirements files. I'll fix later today 🤦🏻‍♀️

[leahecole]

I have been talking to TWs and need to move the Airflow 1 versions to a "legacy samples" folder

[leahecole]

Actually, I'll do the legacy samples thing in a separate PR. I'm ready for you, @dinagraves ! :)