Update `dotnet-buildtools-prereqs-docker` images

[richlander]

We are no longer supporting Ubuntu 18.04 images.

I tried to update them, but there doesn't seem to be matching images for a higher Ubuntu version.

https://github.com/dotnet/machinelearning/blob/70e191b3fae444f6625fdc001071de1e2bd1080b/.vsts-dotnet-ci.yml#L33-L40

The CentOS 8 stream image will soon be EOL. We need to move off of it, too.

https://blog.centos.org/2023/04/end-dates-are-coming-for-centos-stream-8-and-centos-linux-7/

https://github.com/dotnet/machinelearning/blob/70e191b3fae444f6625fdc001071de1e2bd1080b/.vsts-dotnet-ci.yml#L31

I did update your VM images: https://github.com/dotnet/machinelearning/pull/7087