Bump rails gem version to 6.0.3.7

What does this MR do?

It contains multiple security fixes: https://weblog.rubyonrails.org/2021/5/5/Rails-versions-6-1-3-2-6-0-3-7-5-2-4-6-and-5-2-6-have-been-released/

One of them prevent string polymorphic route arguments: https://github.com/rails/rails/commit/c4c21a9f8d7c9c8ca6570bdb82d64e2dc860e62c, that's why we need to fix the existing cases where we're passing String instead of a Symbol to a polymorphic path builder