Track sts tag in addition to latest

[notjosh]

As of 2.20.x, Portainer has split the releases into LTS (long term support) and STS (short term support), see their blog entry about it.

This means that as of today, latest still points to 2.19.x. In order to get the actual latest (i.e. 2.20.x), you need to use the sts tag.

I think it's as simple as updating ci_cd.js to allow also sts tags as part of the build, but I'm not sure if there's gonna be some cases where latest being an old version causes the script to not rebuild it correctly/consistently.