Context: https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=4855000&view=logs&j=291ea0a5-8698-5441-8000-f7fa1e7709b7&t=6f8f21ba-ac47-54c3-4dfb-6d90f0f1c309

The macOS Designer Tests fail to *build*:

    /Users/runner/work/1/s/UITools/Designer/external/Xamarin.PropertyEditing/Xamarin.PropertyEditing.Mac/Controls/BaseDateTimeEditorControl.cs(2,7):
    error CS0246: The type or namespace name 'AppKit' could not be found (are you missing a using directive or an assembly reference?)
    [/Users/runner/work/1/s/UITools/Designer/external/Xamarin.PropertyEditing/Xamarin.PropertyEditing.Mac/Xamarin.PropertyEditing.Mac.csproj]
    …plus 931 other errors…

which is a tad "odd" in that the macS Designer Tests build and run
successfully for both d16-10 and main branches, but not d16-11.

My conjecture is that "something changed" in UITools/d16-11 to break
the macOS build.

Update the "Clone and update UITools" step for
`designer_integration_mac` and explicitly checkout the d16-10 branch.
Let's see if that improves things.

Context: https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=4857598&view=logs&j=fb3518d4-6415-56d8-56e7-3e01113bd696&t=339edcd9-c55f-51d5-0409-e3f1c4277c4d
Context: https://discord.com/channels/732297728826277939/732297837953679412/852637194221125683
Context: dotnet#6014

Looks like dotnet#6014 attempted to bump to a system
mono, and that bump broke ~everything:

	226) Error : Xamarin.Android.Build.Tests.SingleProjectTest.AndroidManifestProperties
	System.IO.FileNotFoundException : Could not load file or assembly 'Microsoft.NET.StringTools, Version=1.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified.
	   at Microsoft.Build.Evaluation.Expander`2.MetadataExpander.ExpandMetadataLeaveEscaped(String expression, IMetadataTable metadata, ExpanderOptions options, IElementLocation elementLocation)
	   at Microsoft.Build.Evaluation.Expander`2.ExpandIntoStringLeaveEscaped(String expression, ExpanderOptions options, IElementLocation elementLocation) in /Users/builder/jenkins/workspace/build-package-osx-mono/2020-02/external/bockbuild/builds/msbuild-15/src/Build/Evaluation/Expander.cs:line 414
	   …

Set `$(MonoRequiredMaximumVersion)` to 6.12.0.137, to *constrain*
which system mono we use.  Hopefully this will "un-bork" CI.