Update project_level_notifications to handle unknown value validation

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

This MR aligns the validation of project_level_notifications to the global_level_notifications changes that were recently made