Split publishing on a per-version basis

Currently publishing of .NET container images is done by publishing all .NET versions together. This is because there can be dependencies amongst the different versions via the `runtime-deps` base image.

But this is problematic because any issue that pops up for a given .NET version will block publishing as a whole, affecting the publishing of _all_ versions. For example, there have been cases in the past where something was discovered on release day that prevented the release of a Preview version and caused it to be delayed by several days. This would block the publishing of the .NET versions that are in servicing and would impact the release of security fixes. There can be no delay for this. In order to work around this, the preview changes would have to be reverted and all the images rebuilt. This in itself increases the delay of publishing those images that, ideally, should have been unaffected by the issue.