Retire prow `image-builder` since `kaniko` is archived

[oliver-goetz]

image-builder in prow is using kaniko which is archived (see here)
Since we build everything else with Github workflows, we should retire the tool and switch to Github workflows.
This also harmonizes processes in the Gardener organisation.

• Build prow and copy-images images with Github workflows #5485
• Create dedicated build test workflows #5506
• Replace kaniko based image build tests with moby/buildkit based ones #5540
• Introduce a build test script which is able to build multiple targets of a Dockerfile #5543
• Switch building golang-test image to a Github workflow gardener#14156
• Build krte image with a Github Workflow

[oliver-goetz]

/kind technical-debt