Add project, group and ~~instance~~-level variable changes to audit events

Problem

We already delivered group/project-level CI/CD variable activities for three events: group variable additions, removals, and protected state changes.

This issue is looking to capture the event of variables' changes, for examples, changes in the values of already existing variables, currently we are not tracking this event.

Proposal

• Log activity/changes in Project > Settings > CI/CD > Variables
• Log this activity to the project-level audit event screen

Implementation Proposal

• Update the AuditVariableChangeService class to handle :update actions.
  • Currently, it looks like the only audited change is the protected status. We'll need to add support for the other attributes too.
• The fix will automatically apply to group and instance level variables too.

Who is the buyer?

GitLab Premium