Unable to trigger "Show all" button action for <MultiList/> via keyboard

**Affected Projects**

React

**Library Version:** x.y.z

3.39.1

**Describe the bug**

In my `` component, I have `showLoadMore` set to `true`. When tabbing through the `` component via keyboard, the "Show all" button is not focusable via keyboard.

So I set an attribute of `tabIndex` to 0 for that button, and it made it focusable successfully. However, even so, pressing Enter/ space did not trigger the same action (e.g. expand 4 more options) as it should with a mouse.

In other words, I'm unable to trigger "Show all" action in component via keyboard only.

**To Reproduce**

Steps to reproduce the behavior:
1. Use the keyboard "tab" key to tab through a component
2. After tabbing through all the checkbox options, try tabbing on the "Show all" button
3. Bug: "Show all" button is not focusable by keyboard. I'm unable to trigger "Show all" action in component via keyboard only.

**Expected behavior**

Users should be able to trigger "Show all" button action in component via keyboard only.

**Screenshots**

1. Default - unable to focus on "Show more" button via keyboard
2. After me trying to set `tabIndex` attribute - still unable to trigger the button action via keyboard
3. How I set attribute tabIndex to MultiList component

**Default - unable to focus on "Show more" button via keyboard**

https://user-images.githubusercontent.com/56893094/226723857-a6bc4f43-ff7b-4e49-8e78-c61a2284ff63.mov

**After me trying to set `tabIndex` attribute - still unable to trigger the button action via keyboard**

https://user-images.githubusercontent.com/56893094/226723977-85af2e8c-4a74-46f3-8b63-beb691e758af.mov

**How i set attribute tabIndex to MultiList component**

**Desktop (please complete the following information):**
- OS: [e.g. iOS]: macOS
- Browser [e.g. chrome, safari] Chrome
- Version [e.g. 22] Version 110.0.5481.177 (Official Build) (x86_64)

**Additional context**

Beginne damit, die React MultiList-Implementierung und das showLoadMore/“Show all”-Steuerelement zu lokalisieren, und reproduziere dann den beschriebenen Tab- und Enter/Space-Ablauf. Die Aufgabe ist erledigt, wenn das Steuerelement per Tastatur erreichbar ist und Enter oder Space die verbleibenden Optionen erweitert, wobei das bestehende Mausverhalten erhalten bleibt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.