Switch to non-EOL .NET Core SDKs in the CI

[Sergio0694]

Summary

The CI is currently showing this warning:

Install NBGV tool (DotNetCoreCLI)

.NET Core SDK/runtime 2.2 and 3.0 are now End of Line(EOL) and have been removed from all agents. If you're using these SDK/runtimes, kindly upgrade to newer versions which are not EOL, or else use UseDotNet task to install the requied version.

DotNetCoreCLI task

Assigning @azchohfi just as a ping, as you're the CI guru 😄

Steps to Reproduce

Steps to reproduce the behavior:

1. Open any CI run page, eg. this one

Screenshots

[azchohfi]

The notifications package still targets 3.0. We should move it to 3.1.
https://github.com/windows-toolkit/WindowsCommunityToolkit/blob/5d03cac977f8e75a91b277066d406458c69f223f/Microsoft.Toolkit.Uwp.Notifications/Microsoft.Toolkit.Uwp.Notifications.csproj#L4
I think that is the only project that targets and sdk that is now EOL.
@Sergio0694 did you see anything else targeting an EOL sdk? Maybe one of the test projects...

This is what installs the last SDK that supported 3.0:
https://github.com/windows-toolkit/WindowsCommunityToolkit/blob/5d03cac977f8e75a91b277066d406458c69f223f/azure-pipelines.yml#L28-L33

A PR here would be very simple, and if it passes, it should be quite strait forward to merge it.

@michael-hawker I don't think we have any scenario that would require us to support 3.0 or older, right? It is not supported anymore, so we should not support it either.

[azchohfi]

I'll send a PR.

[michael-hawker]

Yeah, especially as EOL, we should move forward, I'm not aware of any reasons for us to need to support specific versions of .NET Core as we only target .NET Standard for the library. It's really all about pipeline/build/CI to build the thing vs. end-developer dependencies.

I guess we just missed this one in the last update, wish we could centralize these versions across projects/cake....

Thanks Alex!

[Sergio0694]

Hey @azchohfi - sorry for the delay!
Yeah I just checked and both the test projects for the HighPerformance package and the base toolkit packages only target .NET Core 2.1 and 3.1, I think we had already upgraded them while working on #3167, during the whole restructuring 😊