Update MacOS version to 12

[jander-msft]

Summary

It seems that .NET 9 Preview 4 requires MacOS 12 at minimum to run. When installing newer Preview 4 candidates, running dotnet will fail with:

Failed to load /Users/runner/work/1/s/.dotnet/host/fxr/9.0.0-preview.4.24223.11/libhostfxr.dylib, error: dlopen(/Users/runner/work/1/s/.dotnet/host/fxr/9.0.0-preview.4.24223.11/libhostfxr.dylib, 1): Symbol not found: __ZNSt3__113basic_filebufIcNS_11char_traitsIcEEE4openEPKcj
  Referenced from: /Users/runner/work/1/s/.dotnet/host/fxr/9.0.0-preview.4.24223.11/libhostfxr.dylib (which was built for Mac OS X 12.0)
  Expected in: /usr/lib/libc++.1.dylib

Update the MacOS version for pipeline jobs to 12.

Release Notes Entry

[akoeplinger]

@jander-msft @schmittjoseph FYI Azure DevOps will remove the macos-11 image in about two weeks so this needs to be backported to servicing branches too and also fixed here: https://github.com/dotnet/dotnet-monitor/blob/main/eng/pipelines/jobs/build.yml#L73

[jander-msft]

@akoeplinger, what is going to be done for .NET 6 runtime here? https://github.com/dotnet/runtime/blob/release/6.0/eng/pipelines/common/xplat-setup.yml#L124

We'll probably follow the same solution.

[jander-msft]

/backport to main

[github-actions]

Started backporting to main: https://github.com/dotnet/dotnet-monitor/actions/runs/9552611459

[jander-msft]

/backport to release/8.0

[github-actions]

Started backporting to release/8.0: https://github.com/dotnet/dotnet-monitor/actions/runs/9552620316

[jander-msft]

/backport to release/6.x

[github-actions]

Started backporting to release/6.x: https://github.com/dotnet/dotnet-monitor/actions/runs/9552699295

[akoeplinger]

@jander-msft yes, we need to fix it everywhere :) I checked and dotnet/runtime#103366 bumps to macos-12