[ci] remove uses of azurecr.io for CI images

[jameslamb]

This repo currently uses a container image hosted on an Azure Container Registry repo that @shiyu1994 set up to help us work around some DockerHub rate-limiting issues #6866 (comment)

This PR proposes removing uses of it, so Microsoft can remove that image repo: #6949 (comment)

This is the last bit of Microsoft infrastructure to remove (#7187).

Notes for reviewers

Where did this new image come from?

I pulled the existing one from DocekrHub and pushed it as a public GHCR package here.

docker pull lightgbm/vsts-agent:manylinux_2_28_x86_64

gh auth refresh --scopes 'write:packages'
echo $(gh auth token) | docker login ghcr.io -u jameslamb --password-stdin

docker tag \
  lightgbm/vsts-agent:manylinux_2_28_x86_64 \
  ghcr.io/lightgbm-org/lightgbm-vsts-agent:manylinux_2_28_x86_64

docker push \
  ghcr.io/lightgbm-org/lightgbm-vsts-agent:manylinux_2_28_x86_64

The images on DockerHub were built from here: https://github.com/guolinke/lightgbm-ci-docker

What does this cost?

Nothing. Per https://docs.github.com/en/billing/concepts/product-billing/github-packages

GitHub Packages usage is free for public packages. In addition, data transferred in from any source is free.