Release planning: v1.20.0

[flavorjones]

See https://github.com/sparklemotion/nokogiri/milestone/34

• Add native support for ARM Windows: Add binary gem for platform aarch64-mingw-ucrt #3530
• Update libgumbo with the latest HTML attribute escaping rules changes
  • [html5 spec] Update attribute serialization to escape < and > #3529
  • HTML5 proposal and spec change are linked from that issue, but TLDR all the major browsers have already adopted this behavior
• Upgrade to libxml 1.15
  • https://gitlab.gnome.org/GNOME/libxml2/-/releases/v2.15.0
  • The big change in this version is to attribute escaping is aligned with the change noted above, except in URLs. We'll need to create a patch to get consistent behavior.
• Upgrade to libxslt 1.1.45
  • Which has security fixes in it, but requires libxml >= 1.15.1
• Thanks sponsors in the release notes