[Release train] Automate the creation of test package

[jbordonado]

Description

The release doc currently mentions that the test package should be created each Monday with a manual run of Create a GH pre-release workflow.

Should we automate this with a CRON schedule? Same one (Sunday 12 UTC) as the release package is probably enough?

Acceptance criteria

• Test package is created and ready each Monday
• Slack notification is sent to #wcpay
  • which test package was created and link to the tag
  • diff between this package and the previous one (either previous test package or previous release version if week 1)
  • Reminder to update the WooCommerce Payments plugin prerelease with the new test tag for TumblrPay tests
• Release doc reflects the changes