Release v21

RELEASE

@@ -1,9 +1,5 @@
-v20
+v21
 
 ## Changes
 
-* Adds the Nix versions 2.12.0, 2.11.1 and 2.3.16
-
-* Changes the default Nix version from 2.11.0 to 2.12.0
-
-* Removes the Nix versions 2.11.0, 2.9.1, 2.6.0, 2.3.15, 2.3.14, 2.3.12, 2.3.10, 2.3.7. All of these versions has been obsoleted by newer minor revisions.
+* `nix-quick-install-action` now supports installation into a pre-existing `/nix` directory. This means that you for example can cache the contents of `/nix` between different jobs or different runs of one job, for performance gains. `nix-quick-install-action` will not remove any existing content found in the Nix database or store. For this to work, the pre-existing `/nix` directory must be writeable by the job runner user.