Audit variable logs

What does this MR do?

Re-revert of !35977 (merged)

Adds audit logging for the following events:

• Group variable addition
• Group variable removal
• Group variable protection state change

This is the first iteration and covers only group variables. I'll create follow-up MR that extends this functionality for project variables after this is merged.

Related #30857 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• N/A Database guides
• Separation of EE specific content