Alert widget in monitoring dashboard uses gon instead of vue mixin

Summary

The alert widget component uses gon.features instead of a Vue's glFeatureFlagsMixin.

More details about how to use the mixin here https://docs.gitlab.com/ee/development/fe_guide/vue.html#accessing-feature-flags

Improvements

Better coding standards and consistency across the codebase.

Risks

None

Involved components

https://gitlab.com/gitlab-org/gitlab/blob/master/ee/app/assets/javascripts/monitoring/components/alert_widget_form.vue#L103