Code block buttons lack tooltips

### Steps to reproduce

Hover over any code block, and look at the expand/copy buttons

### Outcome

#### What did you expect?

The buttons should have tooltips and `aria-label`s.

#### What happened instead?

There is no textual indication of what the buttons do, which hurts accessibility.

### Operating system

NixOS unstable

### Browser information

Firefox 96.0.2

### URL for webapp

develop.element.io

### Application version

Element version: 0041ad7230a6-react-bb03298e6dad-js-9c242a9ce6d9 Olm version: 3.2.8

### Homeserver

Synapse 1.51.0

### Will you send logs?

No