[Bug] GitHub Action environment using preview SDK version for all targets.

[filzrev]

Describe the bug
CI build, test and pack operations are executed by using preview version of .NET SDK.

To Reproduce
GitHub action logs indicate all build/tests(net6.0/new7.0/new8.0) operations are executed on NET 8.0 SDK preview2
(latest installed SDK).

https://github.com/dotnet/docfx/actions/runs/5450482248/jobs/9915792025

Microsoft (R) Test Execution Command Line Tool Version 17.6.0-preview-20230223-05 (x64)

How to fix problems

When targeting net6.0 and net7.0.
Build, test and pack operations should be executed on latest stable SDK (.NET 7).
This behavior can be configured by creating following global.json temporary on CI.

{
  "sdk": {
    "allowPrerelease": false
  }
}

Additional context

Please update CI environment .NET 8 SDK to preview5 (8.0.100-preview.5.23303.2)
If there is no compatibility problems exists.

https://github.com/dotnet/docfx/blob/main/.github/actions/build/action.yml#L12-L15
https://github.com/dotnet/core/blob/main/release-notes/8.0/preview/8.0.0-preview.5.md

[filzrev]

This issue is resolved by #9424 that use .NET8 RTM on CI environment.

To support upcoming .NET 9 Preview.
I thought it needs dedicated CI build pipeline for preview version.