Initial PR for issue #674

[jasnow]

Here is the initial PR for this issue #674

Known assumptions

• Only covers "url:" and "related:" (not "cve:" values) fields.
• Assumes PR Removed removed/duplicate URL #934 has been merged.
• Assumes http statuses (200, 301, 302, 308, and 403) are OKAY. Feedback is welcome.
• Only covers advisories from 1/1/2025 going forward. Lots of cleanup with older advisories plus "rake" run is MUCH slower for all advisories.

[postmodern]

I previously decided to avoid checking the URLs as it would slow down the CI. A partial check might seem pragmatic, but then it allows dead URLs in older advisories.