Update omniauth-saml to 1.6.0 to address a security vulnerability in ruby-saml

Updates `omniauth-saml` to bring in the new `ruby-saml` dependency that addresses [CVE-2016-5697](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-5697)

Fixes #19206

See merge request !4951
(cherry picked from commit c3a8b252)