Skip CSRF check on SAML failure endpoint

What does this MR do?

Whitelists :failure endpoint on OmniauthCallbacksController from CSRF checks as this is called when we wish to display an error message or count failed login attempts.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56574

Might also help with https://gitlab.com/gitlab-org/gitlab-ce/issues/48961 and https://gitlab.com/gitlab-org/gitlab-ce/issues/44913

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer