Remove Ubuntu 18.04 from libraries Helix queues

[CarnaViire]

Main goal was to remove prereqs:ubuntu-18.04-helix-arm64v8, and there was even prereqs:ubuntu-22.04-helix-arm64v8 already in place just above, so I just left one line instead of two.

I've also risked updating from Ubuntu.1804.ArmArch.Open helix machine to Ubuntu.2004.Armarch.Open because I see this issue closed dotnet/arcade#10932. If it won't fly, I'll roll it back.

I only limited the change to Libraries legs.

cc @wfurt

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Issue Details

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Main goal was to remove prereqs:ubuntu-18.04-helix-arm64v8, and there was even prereqs:ubuntu-22.04-helix-arm64v8 already in place just above, so I just left one line instead of two.

I've also risked updating from Ubuntu.1804.ArmArch.Open helix machine to Ubuntu.2004.Armarch.Open because I see this issue closed dotnet/arcade#10932. If it won't fly, I'll roll it back.

I only limited the change to Libraries legs.

cc @wfurt

Author:	CarnaViire
Assignees:	CarnaViire
Labels:	area-Infrastructure-libraries
Milestone:	-

[CarnaViire]

Ubuntu.2004.Armarch.Open queue and prereqs:ubuntu-22.04-helix-arm64v8 image seem to be working fine, e.g.

https://helixre8s23ayyeko0k025g8.blob.core.windows.net/dotnet-runtime-refs-pull-94293-merge-a366bc12dedb4b4bb6/System.Net.Quic.Functional.Tests/1/console.5b994ab1.log?helixlogtype=result
Console log: 'System.Net.Quic.Functional.Tests' from job a366bc12-dedb-4b4b-b680-611df092739e (ubuntu.2004.armarch.open) using docker image mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-22.04-helix-arm64v8 on a00068L

https://helixre8s23ayyeko0k025g8.blob.core.windows.net/dotnet-runtime-refs-pull-94293-merge-325f6cbe283a42cebb/System.Net.Quic.Functional.Tests/1/console.4bd19c21.log?helixlogtype=result
Console log: 'System.Net.Quic.Functional.Tests' from job 325f6cbe-283a-42ce-bb7a-d69420eedac6 (ubuntu.2004.armarch.open) using docker image mcr.microsoft.com/dotnet-buildtools/prereqs:debian-11-helix-arm64v8 on a00068M

[wfurt]

LGTM

[carlossanlop]

cc @dougbu who has been overseeing these kinds of changes in the last weeks.

[tmds]

I don't know the context for this removal, I want to check this isn't contrary my assumption that .NET 9 will be built on Ubuntu 18.04 (per #91826 (comment))?

[wfurt]

18.04 is EOL https://ubuntu.com/blog/ubuntu-18-04-eol-for-devices so I'm not sure what we really want with that. @jkotas or @richlander may have some more insight.

In this particular case this PR only changes the underlying queue while tests still running in the container e.g. Debian or Alpine.

[jkotas]

I don't know the context for this removal, I want to check this isn't contrary my assumption that .NET 9 will be built on Ubuntu 18.04

There is a difference between what we support and test and what we build against. For example, .NET 8 is built against Ubuntu 16.04 but it is supported (by Microsoft) on Ubuntu 20.04+ only.