[update] sysutils/telegraf 1.20.3

hi,

here's a long overdue update to telegraf 1.20.3:
https://github.com/influxdata/telegraf/releases/tag/v1.20.3

the version we ship so far is from beginning of 2020, and this new
version seems to work ootb on my installs (some current, some running
7.0), using the following input plugins: 'disk diskio mem net netstat
processes snmp swap system' and writing to influxdb on 7.0.

portswise, the port was converted to use go module (painful) and relies
on the patch from the go.mod.mk thread. I've had to disable two input
plugins for https://github.com/influxdata/telegraf/issues/10034 and
https://github.com/influxdata/telegraf/issues/10035.

feedback welcome, i'll probably commit that in some days. Need to give
more time to influxdb too, an update to 1.9.5 is in the works but the
build fails without giving details.

Landry