[Doc]: Outdated QT_API docs

### Documentation Link

https://github.com/matplotlib/matplotlib/blob/b09aad279b5dcfc49dcf43e0b064eee664ddaf68/doc/users/explain/backends.rst?plain=1#L238

### Problem

The docs for QT_API don't mention qt6.

Probably a good idea to fix that pre 3.5 release?

### Suggested improvement

Mention qt6

### Matplotlib Version

master (pre 3.5)

### Matplotlib documentation version

master (pre 3.5)