Improve Accordion Item Focus State

What does this MR do?

This MR improves the accordion item (ee/app/assets/javascripts/vue_shared/components/accordion/accordion_item.vue) focus state. Currently, when focusing on an item, the native browser outline is shown. However, it should rather show no outline (which makes the component look "unfinished and unpolished", see #214538 (closed)) and instead use the background color as indicator for focus.

The problem with the current state is that the accordion item has a button and a .list-group-item-action nested inside the button. But that class should style a button or anchor, so I moved the class to the button which 1. removes the native outline and 2. adds the correct focus behaviour.

Screenshots

Before

After

Closed: Initial

Closed: Focus / Hover

Open: Focus / Hover

Open: Unfocused

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #214538 (closed)