[Enterprise Search] Improve flash messages screen reader UX (#103412)

* Remove role region on flash messages

- just `aria-live` is enough for screen readers to read it out, and `role` was causing "Flash messages" to get read out loud repeatedly between page navigation even when empty which was annoying and not good

* Further a11y attribute recommendations from @myasonik

x-pack/plugins/enterprise_search/public/applications/shared/flash_messages/flash_messages.tsx

@@ -10,7 +10,6 @@ import React, { Fragment } from 'react';
 import { useValues, useActions } from 'kea';
 
 import { EuiCallOut, EuiSpacer, EuiGlobalToastList } from '@elastic/eui';
-import { i18n } from '@kbn/i18n';
 
 import { FLASH_MESSAGE_TYPES, DEFAULT_TOAST_TIMEOUT } from './constants';
 import { FlashMessagesLogic } from './flash_messages_logic';
@@ -19,14 +18,7 @@ export const FlashMessages: React.FC = ({ children }) => {
   const { messages } = useValues(FlashMessagesLogic);
 
   return (
-    <div
-      aria-live="polite"
-      role="region"
-      aria-label={i18n.translate('xpack.enterpriseSearch.flashMessages.regionAriaLabel', {
-        defaultMessage: 'Flash messages',
-      })}
-      data-test-subj="FlashMessages"
-    >
+    <div role="alert" aria-live="polite" data-test-subj="FlashMessages">
       {messages.map(({ type, message, description }, index) => (
         <Fragment key={index}>
           <EuiCallOut