[FEATURE REQ] Upgrade DotNetCoreRuntimeVersion to latest (3.1.2)

[chidozieononiwu]

Upgrade DotNetCoreRuntimeVersion in eng\pipelines\templates\variables\globals.yml to the latest available on our build agents i.e. 3.1.2.

@AlexGhiondea Should we move past Net core netcore 2.1 or stay there and add a netcore 3.x test leg as well?

[heaths]

Might be a good idea, but I have concerns about timing and resources. This will incur additional Azure resources, so right now under current use guidance, it might not be a good idea.

Even still, we do have some live tests that take quite a while and we can only parallelize so much because of limits. If we increase the number of platforms/frameworks to test, we will either need to increase those limits, or increase any timeouts on pipelines we have and expect longer pipelines.

[weshaggard]

@AlexGhiondea can you or someone on your team advise on what version of .NET Core we want to test?

[kurtzeborn]

We just added runtime support for .NET 5. We don't need to track this one any more.