Switch to v-safe-html for notes in alert_details component

Resolves #241843 (closed)

What does this MR do and why?

Replace v-html with v-safe-html by using GlSafeHTMLDirective for app/assets/javascripts/vue_shared/alert_details/components/system_notes/system_note.vue

How to reproduce

1. Create some alerts for a project
2. Navigate to a project with alerts already triggered
3. Navigate to an alerts detail page and view the Activity Feed tab

Screenshots

Before	After