Remove EuiComboBox down arrow button from tab order

[cjcenizal]

I don't think the accessibility of this component is improved by making this arrow button focusable, since you can tab into the component and use the down arrow and Esc to open and close the suggestions list, respectively. Keeping it in the tab order means that users need to hit Tab an extra time to exit the component. I'd consider it an accessibility improvement if we remove it from tab order.

[myasonik]

Attaching to #2841 which mentions this

For whatever it's worth, the aria practices guidelines also exclude the down arrow

[github-actions]

👋 Hey there. This issue hasn't had any activity for 180 days. We'll automatically close it if that trend continues for another week. If you feel this issue is still valid and needs attention please let us know with a comment.

[github-actions]

❌ We're automatically closing this issue due to lack of activity. Please comment if you feel this was done in error.

[cjcenizal]

Still valid

[1Copenut]

@cjcenizal I'm going to add this to my work on #5636. Thanks for keeping it active.