Upgrade ruby-saml version

What does this MR do and why?

• Just doing some currency work to make sure we are on latest versions
• #409879 (closed)
• No big changes in diff: https://my.diffend.io/gems/ruby-saml/1.13.0/1.15.0

How to set up and validate locally

1. Set up Instance SAML locally: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/saml.md#instance-saml-with-docker
2. Confirm that SAML login still works

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.