Stop explicitly installing bundler.

Use version of bundler included with Ruby docker image, rather than
installing a specific version.

.gitlab-ci.yml

 image: ruby:3.3
 
-before_script:
-  - gem install bundler:2.3.26
-
 pages:
   script:
-    - bundle _2.3.26_ install
+    - bundle install
     - bundle exec jekyll build -d public
   tags:
     - docker