[automated] Merge branch 'release/2.1' => 'release/3.1' (#3867)

* Update branding to 2.1.27 (#3842)

* Update branding to 2.1.27

* Fix patchconfig, baseline val

* Update SDK and baseline

* [release/2.1] Update branding to 2.1.28 (#3855)

* Update branding to 2.1.28

* Bump incoming package patch versions
- e.g. `$(MicrosoftNETCoreAppPackageVersion)`
- updated xunit.assert and xunit.extensibility.execution minor versions
    - align with xunit package version

* Use Ubuntu 18.04 build agents
- set locale consistently on all platforms
    - default locale on newer agents is unloved `C.UTF-8`

* !flaky fix! Double compaction timeouts

* [release/2.1] Credentials fix (#3859)

- inspired by dotnet/runtime#50230
- cherry-picked from 1594096

* [release/2.1] Bump some versions (#3866)

* Move to latest KoreBuild / .NET SDK

* Update baselines
- did not ship any C# packages in 2.1.27
- re-enable baseline validation

* Bump Microsoft.NETCore.App package version

Co-authored-by: William Godbe <wigodbe@microsoft.com>
Co-authored-by: John Luo <johluo@microsoft.com>
Co-authored-by: Doug Bunting <6431421+dougbu@users.noreply.github.com>
Co-authored-by: Sebastien Ros <sebastienros@gmail.com>