Run the Upgrade Develop Version Tests workflow on a schedule

[desrosj]

This adds a new schedule trigger event for the upgrade-develop-testing.yml workflow that runs every Monday at 00:00 UTC.

The install-testing.yml workflow already has this configured.

While the build job is configured not to run on push events for non-WordPress/wordpress-develop repositories, the upgrade-tests-develop-forks job can currently run for push events in forks because a skipped job reports it's status as "success" even when skipped. The result would be a failing workflow run because the required artifact is not present.

The upgrade-tests-develop-forks job is also updated here to explicitly exclude all push and schedule events.

Note: because the upgrade-testing.yml workflow defaults to testing the most recent release available from w.org, it does not make sense to run regularly because what is being tested only changes after a release is packaged.

Trac ticket: Core-65845

Use of AI Tools

AI assistance: Yes
Tool(s): Claude
Model(s): Sonnet 5
Used for: Initial to create the initial PR.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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