Draft: Simplify complicated node replacement logic

What does this MR do and why?

Simplify complicated node replacement logic in Banzai::Filter::References::ReferenceFilter.

This extra work was necessary because of a bug in Nokogiri::XML::Node#replace! See https://github.com/sparklemotion/nokogiri/issues/3567 and https://github.com/sparklemotion/nokogiri/pull/3568 for a fix.

This MR fails CI because that fix isn't released yet. Assuming it does get merged and released, this MR can be updated to use the new Nokogiri version, at which point it'll be ready for merge.

/cc @digitalmoksha — you might be interested in the two links above, seeing as you had to work around the bug in the first place!