Upgrade omniauth-saml to 2.1.0

What does this MR do and why?

• Diff: https://my.diffend.io/gems/omniauth-saml/2.0.0/2.1.0
• That also updates ruby-saml to 1.12.0, diff: https://my.diffend.io/gems/ruby-saml/1.9.0/1.12.0
• Biggest change in the update is attribute renames:
  • issuer is converted to sp_entity_id
  • idp_sso_target_url is converted to idp_sso_service_url
  • idp_sso_target_url_runtime_params is converted to idp_sso_service_url_runtime_params
• ruby-saml deprecated the old names but they still work via reader methods:
  • sp_entity_id: https://github.com/SAML-Toolkits/ruby-saml/blob/c9685a98a9c00c38af65364e70a7315eb49a2222/lib/onelogin/ruby-saml/settings.rb#L119-L121
  • idp_sso_target_url: https://github.com/SAML-Toolkits/ruby-saml/blob/c9685a98a9c00c38af65364e70a7315eb49a2222/lib/onelogin/ruby-saml/settings.rb#L81-L83
• Updates in ee/app/models/saml_provider.rb ensure that group admins can test settings for group SAML. See issue for failure that occurred last time we attempted this update: #373803 (closed)
• Fixes #374036 (closed)

Screenshots or screen recordings

After clicking "Verify SAML Configuration":

How to set up and validate locally

1. Confirm that instance SAML still works
   1. Set up instance SAML: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/saml.md#instance-saml-with-docker
   2. Confirm that sign up / sign in via SAML still works
2. Confirm that group SAML still works
   1. Set up group SAML: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/saml.md#group-saml
   2. Confirm that sign up / sign in via Group SAML still works (using GitLab single sign-on URL available via https://gdk.test:3443/groups/#GROUP_NAME/-/saml)
3. Confirm that group admin can "Verify SAML Configuration"
   1. Visit https://gdk.test:3443/groups/#GROUP_NAME/-/saml as a group admin after group SAML is configured
   2. Click "Verify SAML Configuration" button
   3. There should be a "SAML Response Output" section on the page that shows that the configuration is valid. Example of what this looks like

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.