[ci] temporarily pin Azure Devops jobs to ubuntu 20.04

[jameslamb]

All Linux_latest jobs on Azure Devops started failing a few days ago, because the ubuntu:latest tag on DockerHub moved from Ubuntu 20.04 to 22.04.

#5169 (comment)
#5169 (comment)

While attempting to fix this on #5169, I ran into the issue from #5106 (#5106 (comment)). I believe I've identified the root cause there, but I expect fixing it to require significant discussion and effort.

For now, to unblock other development in this project, this PR proposes temporarily pinning the Linux_latest jobs back to Ubuntu 20.04.

[StrikerRUS]

Agree! Thanks for working on this!
Should we also pin Ubuntu version for GitHub Actions (they share the same environments with Azure Pipelines)?

[jameslamb]

I don't think we need to yet.

On Azure DevOps, these jobs are running inside an ubuntu:latest docker container.

On GitHub Actions, they run on the ubuntu-latest VM image (not inside a container), which is still on 20.04 according to https://github.com/actions/virtual-environments.

I think that's why we've only seen job failures on Azure DevOps.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.