CI workflow a lil out of date.

[Skenvy]

Looks like the CI workflow hasn't run properly since a dependabot update to nokogiri back in March last year 477ac40 that bumped it to a version that requires ruby >= 3.0.

I assume that the ci build is supposed to match https://github.com/actions/jekyll-build-pages as closely as possible to test the resulting page that the default gh-page workflow will run, so worth mentioning that they updated the ruby version in their image last year;

1. from 2.7.4 to 3.2.2 in Bump pages-gem to 232 actions/jekyll-build-pages#127
2. to 3.3 in Update Ruby to 3.3 actions/jekyll-build-pages#128