helm: support ignore-not-found in undeploy targets

[kiku-jw]

What this does

Adds the same ignore-not-found Makefile option used by Go projects to Helm-scaffolded projects.

This updates the Helm scaffold template and the generated Helm sample Makefile so make uninstall ignore-not-found=true and make undeploy ignore-not-found=true continue deleting remaining resources when one resource is already absent.

Fixes #5195.

Testing

• go test ./internal/plugins/helm/v1/scaffolds/...
• go build -tags=containers_image_openpgp -o build/operator-sdk ./cmd/operator-sdk

I also attempted to run PATH="$PWD/build:$PATH" go run ./hack/generate/samples/generate_testdata.go; it reached Helm sample bundle generation but failed in the generated sample while downloading/extracting kustomize with tar: Error opening archive: Unrecognized archive format, so I restored the generated sample and kept the expected Makefile update only.

[kunalworldwide]

This aligns the Helm plugin's Makefile scaffold with the Go and Ansible plugin scaffolds, which already have ignore-not-found support on uninstall and undeploy targets.

The ifndef guard defaults to false so existing behavior is unchanged unless the user explicitly passes make undeploy ignore-not-found=true. Testdata Makefile is updated consistently.

One thought: the deploy target doesn't get --ignore-not-found — this is correct since kubectl apply doesn't have that flag (it's a delete-only concern). Just confirming the asymmetry is intentional and matches the other plugin scaffolds.

LGTM.