CI - Docker registry image tag hygiene

[tjtelan]

We create a new tag for docker images on every merge to master. The only users of these images are probably just us.

So we need to be a little more proactive with untagging any of old the prerelease tags.

During CI, maybe after we push new a image, we could read the tags from the fluvio image registry, identify other prerelease tags and delete them.

Related to #1003

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(Alternatively, we consider not using them at all and developing a workflow that uses latest. However, there's helm packaging to consider, so any solution needs to align with the ergonomics of helm.)

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