Add scripts to support updating template and stub hashes.

[freakboy3742]

Briefcase v0.4.5 introduces the need to provide hashes for support packages and stub binaries. These are labor intensive to update by hand; so these are some scripts that automate the process of generating those updates.

The code is here rather than in the individual template repositories because there is enough commonality in the code for determining the newest release and updating templates that it's worth sharing.

Usage:

• python scripts/update_stub.py path/to/template - determines the latest stub binary for the given template, updates the stub binary value, and updates the hashes to match
• python scripts/update_support.py path/to/template - determines the latest support packages for the given template, and updates the hashes to match. This means:
  • Github releases of Python-Apple-support for macOS and iOS templates
  • Github releases of astral-sh/Python-build-standalone for Linux templates
  • Python.org embeddable releases for Windows templates.

The intention is that these scripts will eventually be added as GitHub workflows to automatically update support packages (either on a manual trigger, or on a cron cadence)

There's no tests because the code is an internal utility, and failure will become evident fairly quickly when the update script fails.

Also removes the vestigial references to pygame workflows, since those were removed in Briefcase v0.4.5.

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool
  Assisted-by: Claude Sonnet 5