Remove pre-release versions of docker images from GHCR and DockerHub

[xokdvium]

Problem

We are now uploading multi-arch release images to GHCR: https://github.com/NixOS/nix/pkgs/container/nix/versions. We used to upload those only from master (not the case since #14916), so anything prior to that contains only amd64 builds for pre-release versions. Those don't seem to have any traffic at all (~50 downloads for image) and some images were pulled because we were mistakenly tagging those as latest.

DockerHub pre-release uploads have been broken for some time and nobody noticed: #14893.

Proposed solution

Remove all pre-release versions from the registries to avoid user confusion. Dogfooding is accomplished easier by using the flake anyway.

[xokdvium]

From today's nix maintainers call:

• @edolstra Sounds good.