Add test coverage for Windows Server 2012/R2

[xtqqczze]

Windows Server 2012 and 2012 R2 (with Extended Security Updates) are supported by .NET until October 13, 2026.

2012 R2 is only supported via ESU and there are currently no public ESU machines available. We will have to rely on separate validation.

Originally posted by @agocke in dotnet/runtime#112056 (comment)

[missymessa]

@agocke and @xtqqczze what is the request here?

[agocke]

I believe that, if there are machines with 2012 R2 ESU available, we would like to use them for validation. If there are not, this issue can be closed.

[garath]

The windows.81.* Helix queues are actually Windows 2012 R2 under-the-hood (specifically, they're derived from the 2012-R2-Datacenter image in the public Image Gallery). This should meet your requirements, I believe? If not, let's chat more.

[garath]

Assuming this situation meets requirements. If not, please reopen and let's chat!

[xtqqczze]

@agocke If Windows.81.Amd64.Open is actually Windows Server 2012 R2, which is still supported, should dotnet/runtime#112056 be reverted?

[agocke]

It's not the ESU version, which is the only thing in support.