fix: schedule upgrade-python-requirements monthly and correct user_reviewers

[pshiu]

Description

We want the bot to send a success notification to [email protected] so a ticket is created in our Jira system whenever an upgrade PR is created every month.

Previously, #3795 intended to set send_success_notification to true and not use @edx-revenue-tasks as a user_reviewers. However, that would make the created task for a successful run (which requires the followup action of a review) look very similar to a failed PR (which requires a very different followup action of investigation).

This PR changes the cadence of the GitHub Actions job in upgrade-python-requirements.yml to monthly and adds @edx-revenue-tasks as a reviewer instead of @openedx/revenue-squad.

Supporting information

• Jira: REV-2694
• Corrects: https://github.com/openedx/ecommerce/pull/3795

[mphilbrick211]

@colinbrash Looks like this is ready to merge if it looks good to you!