Upgrade DataGateway Solution to .Net 6

[seantleonard]

Summary

Upgrades the .csproj files to build against .Net 6. Also updates the Build pipeline files to use Ubuntu-latest and windows-2022 as they have .NET 6 installed.

Example of System.Text.JsonSerializer operation. This may also address comments relevant to #60

Example: JsonSerializer.SerializeToDocument(value: "{}", inputType: typeof(string));
Docs: https://docs.microsoft.com/dotnet/api/system.text.json.jsonserializer.serializetodocument?view=net-6.0#system-text-json-jsonserializer-serializetodocument(system-object-system-type-system-text-json-jsonserializeroptions)

resolves #154

[seantleonard]

Will investigate pipeline failures.
Pending resolution of actions/runner-images#4424
--> Adding .NET 6 SDK by default to Ubuntu images in Azure Dev Ops, which currently is not the case. May be able to workaround with custom configured use .net core task https://docs.microsoft.com/azure/devops/pipelines/tasks/tool/dotnet-core-tool-installer?view=azure-devops

[jarupatj]

[aaronburtle]

Looks like the failures in the pipeline are just formatting issues. Dotnet format should be able to fix this. This extension might fix it automatically for you on save.
https://marketplace.visualstudio.com/items?itemName=MadsKristensen.CodeCleanupOnSave

[seantleonard]

hey, @Mathos1432 , do you know if we can configure , within the Build-pipelines.yml file, the version of .Net the container is built with? In this example failed task

The Docker step is running: /usr/bin/docker pull mcr.microsoft.com/dotnet/aspnet:5.0. I didn't see an input variable we could use from here, so wondering if there may be different steps that you might know of.

[aaronpowell]

@seantleonard isn't it using https://github.com/Azure/hawaii-gql/blob/main/Dockerfile as the Dockerfile for build? You could bump that to .NET 6, or parameterise it if desired.

[seantleonard]

Now pending resolution of dotnet/format#1519 and workaround for manually installing dotnet 6 on Mac runs.

dotnet/format#1433

[Aniruddh25]

Just confirming this is only needed when the imageName is macOS-latest, right as per description?

[seantleonard]

As-is, this will download and install .NET 6.x.x SDK for Cosmos tests on all platforms. The effect seems minimal (7-10seconds) on Linux/Mac, but Windows, the task is now ~45 seconds. Maybe as future optimization step, we could specifically not run this on windows, as it is preinstalled on Win2022 images.

[Aniruddh25]

Looks good, thanks for unblocking this!
Could you please update the PR description with latest before merging?