Add `3-latest` tag to Docker Hub

[desertaxle]

We have a 2-latest tag that points to the latest 2.x image version. We should have the same for 3.x images.

[discdiver]

Relatedly, we should tell users about the existence of this image once it's been created in the docs at https://docs-3.prefect.io/3.0rc/deploy/dynamic-infra/deploy-docker#prefect-maintained-docker-images