Merge `a11y` and `component/accessibility` issue labels

### Confirmation

- [x] I am a [maintainer](https://github.com/orgs/electron/people) of the Electron project. (If not, please create a [different issue type](https://github.com/electron/electron/issues/new/).)

### Description

We current have two different issue labels serving the same purpose:

- a11y
- component/accessibility

Goal:

- Pick one that we'll keep (doesn't matter much which one)
- Assign it to all issues that have the other label
- Delete the other label