Bump nokogiri to 1.13.9

What does this MR do and why?

This MR bumps nokogiri from v1.13.8 to v1.13.9. Here is a changelog for this release: https://github.com/sparklemotion/nokogiri/compare/v1.13.8...v1.13.9.

This new version of nokogiri upgrades libxml2 to v2.10.3 from v2.9.14, and this MR also updates tests to reflect the difference in how invalid HTML start tags are handled in this upgrade. The tests were previously updated when nokogiri was using libxml2 version v.2.9.14 in !89960 (merged).

From nokogiri MR https://github.com/sparklemotion/nokogiri/pull/2624:

Upstream libxml2 changed how it handles conditional HTML comments, see https://gitlab.gnome.org/GNOME/libxml2/-/issues/380

Relates to !101408 (closed) raised by Renovate Bot.

How to set up and validate locally

We can run the following spec to validate the test changes:

bundle exec spring rspec spec/requests/api/markdown_snapshot_spec.rb -fd

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.