Fix CI: override bundler version instead of using the stale lockfile pin

First run failed: Gemfile.lock's BUNDLED WITH 2.0.2 is incompatible
with Ruby 3.2 (String#untaint was removed in Ruby 3.0). Pinning
bundler to 2.5 in the workflow itself rather than touching the
checked-in lockfile.

Author: blytkerchan

.github/workflows/ci_validate_jekyll.yml

@@ -17,10 +17,14 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v7
 
+      # Gemfile.lock pins BUNDLED WITH 2.0.2, which is incompatible with
+      # Ruby 3.2 (uses String#untaint, removed in Ruby 3.0). Overriding the
+      # bundler version here rather than editing the checked-in lockfile.
       - name: Setup Ruby
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: "3.2"
+          bundler: "2.5"
           bundler-cache: true
 
       - name: Build site