Get versions back to 0.20-DEV

This reverts commit 8ac1fcd0ce.

config.toml

@@ -9,7 +9,7 @@ pluralizeListTitles = false
 [params]
   description = "Documentation of Hugo, a fast and flexible static site generator built with love by spf13, bep and friends in Go"
   author = "Steve Francia (spf13) and friends"
-  release = "0.20.1"
+  release = "0.21-DEV"
 
 [taxonomies]
   tag = "tags"