Use DotNetCoreVersion 6.0.x

We think this was bumped to a specific preview6 version because of:

https://discord.com/channels/732297728826277939/732297837953679412/874959446747533323

    error CS8400: Feature 'interpolated string handlers' is not available in C# 8.0. Please use language version 10.0 or greater.

However, is it still needed?

Author: jonpryor

build-tools/automation/azure-pipelines.yaml

@@ -19,7 +19,7 @@ variables:
   RunningOnCI: true
   Build.Configuration: Release
   MaxJdkVersion: 8
-  DotNetCoreVersion: 6.0.100-preview.6.21355.2
+  DotNetCoreVersion: 6.0.x
   HostedMacImage: macOS-10.15
   HostedWinVS2019: Hosted Windows 2019 with VS2019
   NetCoreTargetFrameworkPathSuffix: -net6.0