Prefect: K8s Agent references old prefect docker image name

[cicdw]

Description

Whenever the kubernetes agent creates a job, it references our old docker image naming conventions in at least on place here. We should audit all such references and update them for our new naming scheme, which can be inferred from our circleci/config.yml file here

cc: @joshmeek

[joshmeek]

Due to the tagging changes we have a couple options around "prefecthq/prefect:{}".format(image_version)

1. Add a check here to use the new tagging conventions for >= 0.7.0 and old simple version convention for < 0.7.0
2. Add a new tag to the old images to follow the new convention in order to avoid a check

[cicdw]

option 2.) seems like the easiest to me - no maintenance burden, just a one time tag / push of a lot of images

[joshmeek]

@cicdw Okay great, I'll make the PR and before it's merged we can tag those old images

[cicdw]

Images for 0.6.7 and 0.6.6 have been built and pushed with the new naming schema.