Improve search box for filter dropdowns on vulnerability page

What does this MR do?

On the vulnerability report for a project, there are filter dropdowns that lets the user filter the vulnerability list:

This MR does the following:

1. When the dropdown is opened, auto focus on the search box if there is one:

Before	After

This matches the behavior of other dropdowns-with-search in GitLab, for example the label selector on the issue page:

2. Change the placeholder text in the dropdown's search box from Filter... to Search.

This matches the placeholder text used in other dropdowns in GitLab (see above screenshot).

3. Emit dropdown-show and dropdown-hide events when the dropdown is opened/closed.

This lets the parent component that uses filter-body.vue to respond to the dropdown opening and closing. These events aren't used in this MR, but will be used in a subsequent MR, so this is preparation work.

4. Hide the autocomplete for the search box.

The autocomplete list hides the dropdown items:

5. Make the search box always show at the top instead of scrolling.

Before	After

How to test locally

1. Follow these steps to populate the vulnerability report: https://gitlab.com/-/snippets/1956676
2. Open ee/app/assets/javascripts/security_dashboard/components/filters/standard_filter.vue and change :show-search-box="showSearchBox" to :show-search-box="true". By default the search box is only shown if there are 20 items or more, this forces it to always show:

<template>
  <filter-body
    ...
    :show-search-box="true"
  >

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

Related to #284471 (closed)