Audit variable changes in projects

What does this MR do?

Adds audit logging for the following events:

• Project variable addition
• Project variable removal
• Project variable protection state change

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
• N/A Separation of EE specific content