Draft: Refactor audit event for Group SAML membership updater

What does this MR do and why?

Adds audit event types for Group SAML membership updater. Refactors files to use Gitlab::Audit::Auditor to build audit events.

This affects the following files:

• ee/lib/gitlab/auth/group_saml/membership_updater.rb

Because this MR adds a call to Gitlab::Audit::Auditor.audit, specs for OmniauthCallbacksController were updated to expect an additional call to audit.

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.

Related to #374107 (closed)