Build: Remove redundnat setup-ruby step

Ubuntu 20.04 comes with Ruby 2.7, same as we were installing.
Ubuntu 22.04 comes with Ruby 3.0, which is supported by Jekyll 4.x
just fine, same as Ruby 3.1 and Ruby 3.2. As long as we don't use
ubuntu-latest for this job, we already control the major+minor
version of Ruby by proxy of choosing the Debian package channel
(i.e. Ubuntu version).

.github/workflows/github-pages.yaml

@@ -7,6 +7,8 @@ on:
 jobs:
   deploy:
     name: Deploy
+    # Comes with Ruby 2.7
+    # https://github.com/actions/runner-images/blob/releases/ubuntu20/20240324/images/ubuntu/Ubuntu2004-Readme.md
     runs-on: ubuntu-20.04
     env:
       DEPLOY_DIR: _site/
@@ -21,13 +23,6 @@ jobs:
           # Clone and checkout existing branch, or initialise with a new and empty branch
           git clone --depth 5 --branch "${DEPLOY_BRANCH}" "https://github.com/${GITHUB_REPOSITORY}.git" . || git init -b "${DEPLOY_BRANCH}"
 
-      - uses: ruby/setup-ruby@v1
-        with:
-          ruby-version: 2.7
-          bundler-cache: true
-          # If your site is in a subdirectory
-          # working-directory: ./
-
       - name: Jekyll build
         # If your site is in a subdirectory
         # working-directory: ./