v0.21.0 release

With thanks to all contributors involved in this release:
- anriha
- elParaguayo
- Jaakko Sirén
- Joao Duarte
- Joao Faria Miranda Duarte
- jwijenbergh
- K1llf0rce
- Lars
- matpi
- mcol
- Michael Benford
- MithicSpirit
- modestcap
- nivgsk
- Piotr Machura
- Yutao Yuan

CHANGELOG

@@ -1,4 +1,4 @@
-Qtile x.x.x, released xxxx-xx-xx:
+Qtile 0.21.0, released 2022-03-23:
     * features
         - Add `lazy.window.center()` command to center a floating window on the screen.
         - Wayland: added power-output-management-v1 protocol support, added idle protocol,