[Doc]: Outdated QT_API docs

[anntzer]

Documentation Link

matplotlib/doc/users/explain/backends.rst

Line 238 in b09aad2

The :envvar:`QT_API` environment variable can be set to either ``pyqt5`` or

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)