Add mutation for AlertManagementAlert status

What does this MR do?

This adds a mutation to change an AlertManagement Alert's status.

This is needed as per the designs #214542 (closed).

Does this MR meet the acceptance criteria?

Conformity

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

Related to #214542 (closed)