Action to assign milestone to the resource

Under conditions we can use labels to filter with labels. Under action we can also use labels to add additional labels.

Under conditions we can use milestone to filter with milestone. However under action we cannot use milestone to assign a milestone.

This is inconsistent and confusing, and there's no error feedback. Here's an example that we have to fix it with a quick action: gitlab-org/quality/triage-ops!2335 (merged)