use Ruby 3.1.4 (was 3.0.5)

I suppose we could use even newer Ruby? at the moment I'm just
motivated by an HTMLProofer upgrade causing:

io-event-1.5.1 requires ruby version >= 3.1

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: '3.0.5'
+          ruby-version: '3.1.4'
           bundler-cache: true
       - name: Bower
         run: |