[build] Bump to Mono with MSBuild 16.10 (#848)

We've been seeing a NuGet restore error when running `make prepare`
on a macOS systems with .NET 6 Preview 4 (or greater) installed:

	Restoring packages for /Users/peter/source/java.interop/tools/logcat-parse/logcat-parse.csproj...
	NU1202: Package Mono.CSharp 4.0.0.143 is not compatible with net6.0 (.NETCoreApp,Version=v6.0). Package Mono.CSharp 4.0.0.143 supports: net45 (.NETFramework,Version=v4.5)
	…
	Errors in /Users/peter/source/java.interop/tools/logcat-parse/logcat-parse.csproj
	    NU1202: Package Mono.CSharp 4.0.0.143 is not compatible with net6.0 (.NETCoreApp,Version=v6.0). Package Mono.CSharp 4.0.0.143 supports: net45 (.NETFramework,Version=v4.5)

This error does not manifest when restoring or building with older
.NET 6 previews.

This issue is no longer present in a newer version of Mono 6.12 which
includes MSBuild 16.10.

Fix the error by provisioning Mono 6.12.0.145.

build-tools/automation/azure-pipelines.yaml

@@ -109,7 +109,7 @@ jobs:
 
   - script: >
       dotnet tool install --global boots &&
-      boots --preview Mono
+      boots https://download.mono-project.com/archive/6.12.0/macos-10-universal/MonoFramework-MDK-6.12.0.145.macos10.xamarin.universal.pkg
     displayName: Install Mono
 
   - script: make prepare CONFIGURATION=$(Build.Configuration) JI_MAX_JDK=$(MaxJdkVersion)