Migrate Helm users using artifactOverrides

[briandealwis]

Tracking bug: #6706

#6949 removes the artifactOverrides field and instead configured Helm to use Skaffold as a post-renderer filter for hydrating the Helm-rendered manifests. One requirement for this to work is that the images in the rendered manifests must correspond to either the build.artifacts[].image or to the final locations of the images (post-default-repo). This may be a breaking change as some users may have been using image defaults that don't correspond to the image names in the build.artifacts[].image and so their deployments may cease to work.

We should convert uses of artifactOverrides into setValues where we populate the keys with the corresponding artifact[].image using the configured Helm strategy.

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An alternative strategy would be to check for this situation in the schema-upgrade. We could invoke helm template and then look for matched and unmatched image names. If we found mismatches, Skaffold could warn the user or abort the upgrade and direct them to do a manual upgrade. Warning seems inadequate as a missed image can result in a broken deployment (either referencing an image that does not exist or that is not the built image), and users may miss the warning. Aborting and requiring an update would entail one-off toil for users, since they would need to examine their charts and skaffold.yaml to see if there is a discrepancy. Since some users won't rewrite their skaffold.yaml — since we silently upgrade from previous schema versions — our upgrade process would also invoke helm template each time, which is not ideal.