scripted-diff: Drop Darwin version for better maintainability

-BEGIN VERIFY SCRIPT-
sed -i 's/darwin19/darwin/g' $(git grep --files-with-matches 'darwin19')
-END VERIFY SCRIPT-

contrib/guix/guix-codesign

@@ -91,7 +91,7 @@ fi
 ################
 
 # Default to building for all supported HOSTs (overridable by environment)
-export HOSTS="${HOSTS:-x86_64-w64-mingw32 x86_64-apple-darwin19}"
+export HOSTS="${HOSTS:-x86_64-w64-mingw32 x86_64-apple-darwin}"
 
 # Usage: distsrc_for_host HOST
 #