Scrollable region should be keyboard accessible

Report from CI automated tests

WCAG warnings in the test class [org.xwiki.ckeditor.test.ui.AllIT]:
__________
Validation in the test method [paragraphQuickAction_en]
Check for [org.xwiki.test.ui.po.editor.WikiEditPage] at http://xwikiweb:8080/xwiki/bin/edit/NestedLocalizationIT/paragraphQuickAction_en/WebHome?form_token=F9UNThJ8gCk908D26q2rZQ&editor=wysiwyg.
Found [1] items

1: Scrollable region must have keyboard access
Description: Ensure elements that have scrollable content are accessible by keyboard
Help URL: https://dequeuniversity.com/rules/axe/4.10/scrollable-region-focusable?application=axeAPI
Help: Scrollable region must have keyboard access
Impact: serious
Tags: cat.keyboard, wcag2a, wcag211, wcag213, TTv5, TT4.a, EN-301-549, EN-9.2.1.1, EN-9.2.1.3

HTML element:
<ul id="cke_73" class="cke_autocomplete_panel cke_autocomplete_opened" role="listbox" aria-label="Select the item to autocomplete." style="z-index: 9997; left: 46px; top: 273.562px;" data-query="/p">
Selector: cke_73
Fix any of the following:
Element should have focusable content
Element should be focusable

When checking it out, it seems that using tabindex for this element will be difficult... It already supports specific functions with Tab. We might want to make an exception to the WCAG rule for this test.

With XWIKI-20909 , this is the only violation of `scrollable-region-focusable` in `xwiki-platform`.

This is kind of related to XWIKI-22493 , and a PR for fixing both at the same time could work.