fix(samples): fix column name in DagModel of Airflow workflow to clean up database (GoogleCloudPlatform#6532)

## Description

Fixes GoogleCloudPlatform#6531 

## Checklist
- [X ] I have followed [Sample Guidelines from AUTHORING_GUIDE.MD](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md)
- [ ] README is updated to include [all relevant information](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#readme-file)
- [ ] **Tests** pass:   `nox -s py-3.6` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [ ] **Lint** pass:   `nox -s lint` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#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)
- [X ] Please **merge** this PR for me once it is approved.
- [ ] This sample adds a new sample directory, and I updated the [CODEOWNERS file](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/.github/CODEOWNERS) with the codeowners for this sample

Author: kosteev

composer/workflows/airflow_db_cleanup.py

@@ -130,7 +130,7 @@
     "keep_last_group_by": None
 }, {
     "airflow_db_model": DagModel,
-    "age_check_column": DagModel.last_scheduler_run,
+    "age_check_column": DagModel.last_parsed_time,  # prior to Airflow 2.0.2 this column was named last_scheduler_run
     "keep_last": False,
     "keep_last_filters": None,
     "keep_last_group_by": None