minor #5545 DX: update checkbashisms (keradus)

This PR was merged into the 2.18 branch.

Discussion
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DX: update checkbashisms

somehow, 2.20 disappeared from https://deb.debian.org/debian/pool/main/d/devscripts/

Commits
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9f536ed04 DX: update checkbashisms

dev-tools/install.sh

@@ -20,7 +20,7 @@ cd "$(dirname "$0")"
 
 mkdir -p bin
 
-VERSION_CB="2.20.5"
+VERSION_CB="2.21.1"
 VERSION_SC="stable"
 
 echo λλλ checkbashisms