[ci] Update azure-pipelines.yaml to Pin .NET 6.0.202 (#986)

Mono uses the .NET 6 reference libraries to build the `net6.0`
binaries with its `msbuild`.  This works for .NET 6 `6.0.202`;
however when .NET 6 is upgraded to `6.0.300` it now fails with:

	/Library/Frameworks/Mono.framework/Versions/6.12.0/lib/mono/msbuild/Current/bin/Microsoft.Common.CurrentVersion.targets(1232,5):
	error MSB3971: The reference assemblies for ".NETFramework,Version=v6.0" were not found.
	You might be using an older .NET SDK to target .NET 5.0 or higher.
	Update Visual Studio and/or your .NET SDK.

Pin our .NET 6 to the working version for now so we can continue to
build successfully on CI.

build-tools/automation/azure-pipelines.yaml

@@ -21,7 +21,7 @@ variables:
   RunningOnCI: true
   Build.Configuration: Release
   MaxJdkVersion: 8
-  DotNetCoreVersion: 6.0.x
+  DotNetCoreVersion: 6.0.202
   1ESWindowsPool: AzurePipelines-EO
   1ESWindowsImage: AzurePipelinesWindows2022compliant
   1ESMacPool: Azure Pipelines