Use "multiple sources" feature for build.ubuntu.* images

[dougbu]

@riarenas' work on #2113 included creation of base images we would prefer weren't required. There's a performance and storage cost associated with every image we generate. That work was however necessary due to our timing and the unavailability of the "multiple sources" feature from Image Factory.

The feature may now be available, according to a comment from @apilosofms in !38607 and the Image Factory v3 API docs. If this means what we hope it means, we should remove the Ubuntu base images, perhaps the non-VS Windows base images, and maybe even the Mariner base image (not sure why that exists in the first place)❔ Then, use the new feature to include artefacts from the central source directly in our custom images. This will also allow us to fairly easily avoid including linux-1es-gpt-prerequisites in a few of our Helix images.

Note

At this time, I have a question in a Teams channel out to @ChulHul confirming availability of the feature and asking how to use it.

Release Note Category

• Feature changes/additions
• Bug fixes
• Internal Infrastructure Improvements

Release Note Description

Changed how we create a few of our custom build images to avoid base images for them. Also deleted the base image definitions we no longer require.