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.

machinelearning/.vsts-dotnet-ci.yml

Lines 33 to 40 in 70e191b

	- container: UbuntuContainer
	image: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-18.04-mlnet
	- container: UbuntuCrossArmContainer
	image: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-18.04-mlnet-cross-arm
	- container: UbuntuCrossArm64Container
	image: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-18.04-mlnet-cross-arm64

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/

machinelearning/.vsts-dotnet-ci.yml

Line 31 in 70e191b

image: mcr.microsoft.com/dotnet-buildtools/prereqs:centos-stream8-mlnet

I did update your VM images: #7087