Update jupyterhub/k8s-image-awaiter Docker tag to v3

This MR contains the following updates:

Package	Type	Update	Change
jupyterhub/k8s-image-awaiter	ironbank-docker	major	2.0.0 -> 3.0.0
jupyterhub/k8s-image-awaiter		major	2.0.0 -> 3.0.0
jupyterhub/k8s-image-awaiter	stage	major	2.0.0 -> 3.0.0

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