Add csv gem for Ruby 3.4

Author: tvdeyen

lib/solidus_dev_support/templates/extension/Gemfile.tt

@@ -31,6 +31,11 @@ if rails_version == '7.0'
   gem 'concurrent-ruby', '< 1.3.5'
 end
 
+if RUBY_VERSION >= '3.4'
+  # Solidus Promotions uses CSV but does not have it as dependency yet.
+  gem 'csv'
+end
+
 # While we still support Ruby < 3 we need to workaround a limitation in
 # the 'async' gem that relies on the latest ruby, since RubyGems doesn't
 # resolve gems based on the required ruby version.