Small improvements to milestone combobox

What does this MR do?

Makes a few small improvements to the milestone combobox component:

1. The search bar is focused when the dropdown is opened
2. The search input debounce time is updated from 100ms to 250ms to be consistent with the RefSelector component that appears on the same page (the Create from field)
3. v-bind="$attrs" is added to the root node to allow parent component to bind event listeners to the GlNewDropdown child inside this component's template
4. A class is added to the root node to assist in RSpec feature tests (being added as part of !39151 (merged))

Screenshots

This MR makes no screenshot-able changes, but here's one just for fun:

Closes #243724 (closed), because this MR covers the test case mentioned in the issue.