[build] Update more NuGet package versions

[jonpryor]

Context: https://dev.azure.com/devdiv/DevDiv/_componentGovernance/112013/alert/2979569?typeId=6338203

Component Governance is a Microsoft internal tool which checks
for known security issues in product dependencies. It is currently
reporting a defect in Xamarin.Android because Java.Interop restores
the System.Net.Http v4.1.0 NuGet package, which contains security
vulnerability CVE-2018-8292:

/s/xamarin-android/external/Java.Interop/packages/system.net.http/4.1.0/system.net.http.4.1.0.nupkg
/s/xamarin-android/external/Java.Interop/packages/system.net.http/4.1.0/system.net.http.nuspec

System.Net.Http v4.1.0 isn't actually used by Java.Interop or
Xamarin.Android, it's just an implicit NuGet dependency of one of the
various NuGet packages which Java.Interop relies on.

The report is "noise".

That said, we should still remove "noise" when possible.

Update most NuGet package versions within Java.Interop to the latest
versions provided by dotnet-public or dotnet-eng (which may not
be the latest versions on NuGet.org).

Exceptions:

• Microsoft.Xml.SgmlReader is not bumped, as the latest version
  introduced an ABI break.

• Microsoft.CodeAnalysis.FxCopAnalyzers v3.3.2 deprecated the
  entire package, introducing additional warnings that I didn't
  want to deal with right now.

• protobuf-net is still not updated, as the latest version causes
  unit test failures; see also PR Bump protobuf-net #878.

In order to better "centralize" NuGet package version information,
instead of having multiple %(PackageReference.Version) values
strewn throughout the codebase, instead have .csproj files contain
@(PackageReference)s that don't contain
%(PackageReference.Version), and instead use %(Update) within
Directory.Build.targets to provide the actual package version:

<!-- .csproj -->
<PackageReference Include="Example" />

<!-- Directory.Build.targets -->
<PackageReference Update="Example" Version="1.0.0" />

This pattern requires that there be no "intervening"
Directory.Build.targets between the .csproj and the root directory;
see also the MSBuild Customize your build documentation.

Rename the existing Directory.Build.targets files to instead use
explicit <Import/>s against a newly added .targets file;
conceptually reverts part of d70e40f and later commits which use
Directory.Build.targets.

[jpobst]

The actual updates get kinda lost in the noise of moving things around, updated nugets:

• HtmlAgilityPack: 1.11.24 -> 1.11.30
• Microsoft.Build.Framework: 16.5.0 -> 16.11.0
• Microsoft.Build.Utilities.Core: 16.5.0 -> 16.11.0
• Microsoft.CodeAnalysis.CSharp: 16.5.0 -> 16.11.0
• Microsoft.NET.Test.Sdk: 16.2.0 -> 16.11.0
• Microsoft.NETFramework.ReferenceAssemblies: 1.0.0 -> 1.0.0
• Mono.Options: 6.6.0.161 -> 6.12.0.148
• nunit: 3.12.0 -> 3.13.2
• NUnit.ConsoleRunner: 3.11.1 -> 3.12.0
• NUnit3TestAdapter: 3.16.1 -> 4.0.0