Pin Ruby 3.3.

Use Ruby 3.3 as default, and remove csv and base64 from Gemfile, as
these should be dependencies of just-the-docs, rather than our site.
Build failure is fixed in main branch of JTD, as of a few hours ago.

.gitlab-ci.yml

-image: ruby
+image: ruby:3.3
 
 before_script:
   - gem install bundler:2.3.26

Gemfile

@@ -2,5 +2,3 @@ source "https://rubygems.org"
 gemspec
 
 gem "webrick", "~> 1.7"
-gem "csv"
-gem "base64"
\ No newline at end of file