Fix CI errors to do with Nokogiri::HTML4

[bagedevimo]

When submitting MRs, I notice we have some flakey specs. These are (probably) due to a rubygem cache on the circleCI runners using an older version of loofah which doesn't have the fixes introduced in 2.12.2 [1].

https://github.com/flavorjones/loofah/blob/main/CHANGELOG.md#2212--2023-05-11

There are a few options to solve this, the simplest is to wipe the CI caches in CircleCI which should force it to resolve new gems, and install the fixed version of Loofah. Another solution would be to check Gemfile.lock into source control, which might have other issues but would mean you can prescribe the version of loofah used in CI.