Rename CI_REGISTRY_IMAGE predefined env var to CI_PROJECT_REGISTRY
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Problem
According to https://docs.gitlab.com/ee/ci/variables/predefined_variables.html, CI_REGISTRY_IMAGE is The address of the project’s Container Registry.. So, it's the path under which images can be hosted on the container registry for a particular project, it's not full path to any particular image (for that we also need the tag name or digest suffix, e.g. $CI_REGISTRY_IMAGE:latest or $CI_REGISTRY_IMAGE@sha256:e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855). However, the name of the variable makes it sound like we're talking about an image, which is not the case. This has caused some confusion among users, as exemplified in !134904 (merged).
Proposal
Consider deprecating CI_REGISTRY_IMAGE and introducing CI_PROJECT_REGISTRY. Note that this is a breaking change which can impact a lot of users and require them to update most of their CI scripts. So extra precaution is required, reason why we should introduce the latter before removing the former.