)

Fixes: #826

Context: #682

Given the following Kotlin code:

	internal class MyClass {
	  public interface MyInterface { }
	}

The default Java representation of this would be:

	public MyClass {
	  public MyInterface { }
	}

In order to prevent this from being bound, our Kotlin fixups
attempted to change the Java representation to:

	private class MyClass {
	  private interface MyInterface { }
	}

However there was a bug preventing the internal interface from being
marked as `private`, because we are setting the visibility on the
parent type's `InnerClassAccessFlags`, *not* the nested type itself.
When we output the XML we use use the declaring class'
`InnerClassAccessFlags`, we instead look at the flags on the nested
type's `.class` file, which was still `public`:

	<class name="MyClass" visibility="private" … />
	<class name="MyClass.MyInterface" visibility="public" … />

This would result in a `generator` warning when trying to bind
`MyClass.MyInterface`, as the parent `MyClass` type is skipped:

	warning BG8604: top ancestor MyClass not found for nested type MyClass.MyInterface

Fix this by finding the appropriate inner class `.class` file and
updating the access flags there, recursively:

	<class name="MyClass" visibility="private" … />
	<class name="MyClass.MyInterface" visibility="private" … />

Note: the [`InnerClasses` Attribute][0] for an inner class may
contain the declaring class.  For example, the `InnerClasses` for
`InternalClassWithNestedInterface$NestedInterface` contains
`InternalClassWithNestedInterface$NestedInterface`.

We must thus protect recursive `HideInternalInnerClass()` invocations
from processing the current type, to avoid infinite recursion.

[0]: https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.6

Context: https://blog.gradle.org/jcenter-shutdown
Context: https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter/

JCenter is shutting down.  It is no longer browsable as of May 1,
and artifacts hosted on JCenter will not be accessible as of
February 1, 2022.

Stop using JCenter, and instead use [Maven Central][0].

[0]: https://search.maven.org/

Context: dotnet/android#5891
Context: dotnet/android@e59f649

xamarin/xamarin-android should imminently be building libraries
targeting `net6.0` instead of `netcoreapp3.1`.

We should be able to do this now, because Mono 6.12.0.137 supports
building both .NET 5.0 and .NET 6.0.

Update all appropriate `$(TargetFrameworks)` values so that `net6.0`
is used instead of `netcoreapp3.1`.

Update `global.json` so that a Prerelease .NET 6 SDK is used, and use
`boots` to install the preview version of Mono which supports
targeting `net6.0`.