Download Boost source from Sourceforge instead of Jfrog to avoid registration wall

[PhilMiller]

Automated downloads from Jfrog started failing all of our CI jobs.

Changes

• Switch from Jfrog download URL to Sourceforge download URL

Testing

This is entirely about getting CI to pass again

Notes

• Ideally, we'd shift toward packaged versions as in Install boost from package manager, not source #698

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• Reviewers requested with the Reviewers tool ➡️

Target Environment support

• Linux
• macOS