software/grafana: version up telegraf v1.21.2

One notable change since previous version is that a new sql input,
that can be used to get metrics from sql queries.

software/grafana/software.cfg

@@ -53,7 +53,7 @@ revision      = v1.8.4-0-gbc8ec4384e
 <= go-git-package
 go.importpath = github.com/influxdata/telegraf
 repository    = https://github.com/influxdata/telegraf
-revision      = v1.17.3-0-g24a552b90b
+revision      = v1.21.2-0-g30d981d3
 
 
 [gowork]