Fix Ruby dependency vulnerabilities

[uppnrise]

What changed

• Updated ActiveSupport, Addressable, Concurrent Ruby, Faraday, and JSON past the minimum safe versions for all 12 issues reported by the Snyk project.
• Updated Nokogiri to 1.19.4 to address additional advisories found by the current Ruby advisory database.
• Added a regression test that prevents the affected gems from being locked below their safe versions.

Why

The github-pages umbrella gem allows vulnerable transitive versions to remain locked even though patched releases satisfy its dependency constraints. Updating only the affected transitive dependencies removes the vulnerabilities without changing the site framework or deployment model.

Impact

No site behavior is intentionally changed. The Jekyll site continues to build with the GitHub Pages dependency set, while the vulnerable Ruby dependencies are replaced with compatible patched releases.

Validation

• bundle exec ruby test/site_test.rb — 11 runs, 115 assertions, 0 failures
• bundle-audit check --update — No vulnerabilities found
• git diff --check — clean