[#69179750] Upgrade collectd to 5.4

Upgrade from 5.3 to 5.4. The main reason for this upgrade is that the
collectd puppet module we use will only collect varnish stats with
collectd >= 5.4.

Change log:
https://collectd.org/wiki/index.php/Version_5.4
Merge pull request #358 from alphagov/upgrade-collectd

[#69179750] Upgrade collectd to 5.4