Use latest bundler if Ruby version is 3 or higher

Previously, it attemps to install bundler 2.4.21 while Ruby version is 3.4.
https://github.com/rails/thor/actions/runs/13782104138/job/38542040661?pr=899

```
  /opt/hostedtoolcache/Ruby/3.4.2/x64/bin/gem install bundler -v 2.4.21
  Successfully installed bundler-2.4.21
  1 gem installed
  Took   0.37 seconds
```

Author: yahonda

.github/workflows/tests.yml

@@ -12,6 +12,6 @@ jobs:
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
-        bundler: ${{ (matrix.ruby_version < '3' && '2.4.21') || 'latest' }}
+        bundler: ${{ (matrix.ruby < '3' && '2.4.21') || 'latest' }}
         bundler-cache: true # runs 'bundle install' and caches installed gems automatically
     - run: bundle exec thor spec