slack-infra: migrate images to registry.k8s.io and fix slack-welcomer container name

[Champbreed]

Migrates slack-welcomer, slack-moderator and slack-post-message deployments from the deprecated gcr.io/k8s-staging-slack-infra registry to registry.k8s.io. Also fixes the slack-welcomer container name which was incorrectly labeled as slack-moderator. Images are already promoted in registry.k8s.io/images/k8s-staging-slack-infra/images.yaml — not promoting new images, only migrating existing ones.

/sig k8s-infra
/cc @upodroid @ameukam

[k8s-ci-robot]

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[Champbreed]

@ameukam slack-moderator-words is excluded pending PR #9216 while slack-event-log is not yet promoted. If this works.

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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• apps/slack-infra/OWNERS

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[Champbreed]

Awaiting #9216 to merge first as these new tags won't resolve at registry.k8s.io until that's promoted.