CI: Use Ruby 3.2 to run linter

Rails 8.0 (the default Rails we use on CI) needs at least
Ruby 3.2. We still support 3.1 and set the linter to check
for Ruby 3.1 syntax.

.github/workflows/lint.yml

@@ -15,7 +15,7 @@ jobs:
       - name: Install Ruby and gems
         uses: ruby/setup-ruby@v1
         with:
-          ruby-version: "3.1"
+          ruby-version: "3.2"
           bundler-cache: true
           rubygems: "latest"
       - name: Lint Ruby files