[ci] Update to VS2022 build agents.

[jpobst]

Now that VS2019 doesn't officially support .NET 6, we are getting lots of warnings trying to build out net6.0 assemblies on VS2019 build agents:

Warning NETSDK1182: Targeting .NET 6.0 in Visual Studio 2019 is not supported.
Warning CS8032: An instance of analyzer System.Text.Json.SourceGeneration.JsonSourceGenerator cannot be created from C:\hostedtoolcache\windows\dotnet\packs\Microsoft.NETCore.App.Ref\6.0.0\analyzers\dotnet\cs\System.Text.Json.SourceGeneration.dll : Could not load file or assembly 'Microsoft.CodeAnalysis, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified..

DevOps now offers VS2022 build agents we can switch to in order to build net6.0 assemblies with no warnings.