Revert "bump x.y.z version to odd y following creation of RELEASE_3_13 branch"

This reverts commit 2e8360d.

Author: benjjneb

DESCRIPTION

@@ -8,7 +8,7 @@ Description: The dada2 package infers exact amplicon sequence variants (ASVs) fr
     removing substitution and chimera errors. Taxonomic classification is available
     via a native implementation of the RDP naive Bayesian classifier, and species-level
     assignment to 16S rRNA gene fragments by exact matching.
-Version: 1.21.0
+Version: 1.20.0
 Date: 2021-05-13
 Maintainer: Benjamin Callahan <benjamin.j.callahan@gmail.com>
 Author: Benjamin Callahan <benjamin.j.callahan@gmail.com>, Paul McMurdie, Susan Holmes