Install a Bundler version that works with Ruby 2.7

Fixes:

> Installing Bundler
>  Using latest Bundler for ruby-2.7.6 because the default Bundler gem is too old for that Ruby version
>   /opt/hostedtoolcache/Ruby/2.7.6/x64/bin/gem install bundler -v ~> 2.0
>   ERROR:  Error installing bundler:
>   	The last version of bundler (~> 2.0) to support your Ruby & RubyGems was 2.4.22. Try installing it with `gem install bundler -v 2.4.22`
>   	bundler requires Ruby version >= 3.0.0. The current ruby version is 2.7.6.219.
>   Took  14.12 seconds

.github/workflows/ruby.yml

@@ -34,5 +34,7 @@ jobs:
       with:
         ruby-version: ${{ matrix.ruby-version }}
         bundler-cache: true # runs 'bundle install' and caches installed gems automatically
+    - name: Install legacy bundler
+      run: gem install bundler -v 2.4.22 # For Ruby 2.7
     - name: Run tests
       run: bundle exec rake