.gitlab.ci.yml: Use default images as build targets

Merged Frieder Schrempf requested to merge feature/revert-ci-build-targets into develop-dunfell

With SWUpdate support added to the default images, we don't need to build separate image recipes to generate *.swu images. Let's revert to the default image-ktn* images.

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Edited by Frieder Schrempf

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