[Rules > Add Elastic rules][SCREEN READER]: Headings must be properly…

… nested for usability (elastic#180358)

Closes: elastic/security-team#8653

## Description

The Add Elastic Rules empty prompt (all rules added) has an H3
improperly nested. This makes for a more difficult screen reader
experience; a large number of screen reader users [primarily navigate by
headings](https://webaim.org/projects/screenreadersurvey9/#finding).

### Steps to recreate

1. Open [Add Elastic
rules](https://kibana.siem.estc.dev/app/security/rules/add_rules)
2. Add all rules to the monitoring table if not already
3. Open [HeadingsMap
extension](https://chromewebstore.google.com/detail/headingsmap/flbjommegcjonpdmenkdiocclhjacmbi?pli=1)
or your preferred screen reader
4. If using a screen reader, traverse the page by headings only to hear
the perceived importance of information

### Screen 

<img width="1651" alt="image"
src="https://github.com/elastic/kibana/assets/20072247/36f4f2a9-a472-4fa1-9c0c-871f2e98415a">

x-pack/plugins/security_solution/public/detection_engine/rule_management_ui/components/rules_table/add_prebuilt_rules_table/add_prebuilt_rules_no_items_message.tsx

@@ -27,7 +27,7 @@ const AddPrebuiltRulesTableNoItemsMessageComponent = () => {
     >
       <EuiFlexItem grow={false}>
         <EuiEmptyPrompt
-          title={<h3>{i18n.NO_RULES_AVAILABLE_FOR_INSTALL}</h3>}
+          title={<h2>{i18n.NO_RULES_AVAILABLE_FOR_INSTALL}</h2>}
           titleSize="s"
           body={i18n.NO_RULES_AVAILABLE_FOR_INSTALL_BODY}
           data-test-subj="noPrebuiltRulesAvailableForInstall"