Update Nokogiri to v1.15.2

The update in libxml2
(https://gitlab.gnome.org/GNOME/libxml2/-/releases/v2.11.0) stopped
escaping ASCII characters in HTML:
https://gitlab.gnome.org/GNOME/libxml2/-/issues/321. Update the
specs and Markdown snapshot accordingly.

Set the libxml2 memory allocator via
`NOKOGIRI_LIBXML_MEMORY_MANAGEMENT=default` to work around Ruby
interpreter bug. This variable can be dropped once
https://github.com/ruby/ruby/pull/7663 ships with a Ruby interpreter
that is used.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/390313

Changelog: changed