Migrate angle icons to chevron in accordion_item.vue

What does this MR do and why?

Migrates angle icons to chevron in accordion_item.vue.

Note: I'm unsure why we have this accordion shared component within the GitLab project 🤔 . Other instances that utilize this component import it from GitLab UI. I believe the Securities Dashboard is the only location this shared component (from this project) is being used currently.

Screenshots or screen recordings

before	after

How to set up and validate locally

1. You may need to manually create a vulnerability on a project from the Security & Compliance > Vulnerability report page. (this is so you will have an expandable section to test)
2. Go to Security & Compliance > Security Dashboard for the parent group of that project.
3. The Project Security status chart is where you will find this change

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #362786 (closed)