Replace Vue component event bus with mitt implementation in standalone vulnerabilities

Summary

Per !30244 (comment 330660605)

What concerns me is we're adding an event bus and in Vue 3 event bus will be removed. I planned to announce a list of 'features-not-to-be-added' on Tuesday on FE call to make our migration to Vue 3 more smooth, and event buses are one of these features.

Improvements

Reasoning for Vue removing the $on method here

Risks

We need to wait until mitt is installed and verified in our code base

Involved components

ee/app/assets/javascripts/vulnerabilities/components/vulnerabilities_event_bus.js

ee/app/assets/javascripts/vulnerabilities/components/footer.vue

ee/app/assets/javascripts/vulnerabilities/components/header.vue

Related to !30244 (merged)