Update Arm build host to Ubuntu 20.04

[richlander]

@janvorli @jkotas @wfurt

[omajid]

Does that mean .NET 7 on arm64 will no longer be supported on RHEL 8?

cc @tmds @leecow

[richlander]

Good call. That isn't the intention.

This is probably the "what do we do when CentOS 7 goes EOL" come early topic. We likely need to target an older glibc. Right?

[tmds]

Yes, RHEL 8 has glibc 2.28.

[richlander]

Is there a recommended pattern on how to do that?

Here are two examples:

• Tensorflow (use modern OS; download foreign old glibc to use at compile-time)
• Python manylinux (build in docker with old OS)

[wfurt]

Do we consider RHEL8 as "modern OS"?. If so, we could use it as base line. That would make it more similar to x64.

[richlander]

We're in the process of switching (for .NET 7+) of using Mariner as our build host. That likely means that we'll need to acquire a foreign glibc to compile with, much like the Tensorflow model, but in a consistent container image, like the Python manylinux model.

Context: dotnet/dotnet-buildtools-prereqs-docker#615

[omajid]

As others have said, removing support for multiple OS's based on this quite recent glibc version is not desired.