Mark API-31 as stable.

[jpobst]

Set API-S as API-31, v12.0, and stable. Bump .NET 6 pack version to 31.0.100.

Notes:

• API-31 requires JDK 11 to function, so bump Legacy CI lanes to use JDK 11 instead of JDK 1.8. (.NET6 lanes already used JDK 11.)
• Various tests updated to expect API-31 results.
• The source .jar used for API-31 documentation is not available yet, filed an issue to track: Bump android.jar documentation source to API-31 #6215.

[jonathanpeppers]

Go ahead and bump this to 31.0.100:

https://github.com/xamarin/xamarin-android/blob/f7d32102cc3ada67aa12cc0beed2cf97226f41b3/Directory.Build.props#L24

Do we know if it will be Android 12? If we don't know yet, we can leave this one alone:

https://github.com/xamarin/xamarin-android/blob/f7d32102cc3ada67aa12cc0beed2cf97226f41b3/Directory.Build.props#L16

[jonathanpeppers]

It doesn't look like 823fec5 fixed the issue with binding projects.

It's tough because these tests pass for me locally somehow...

[jonpryor]

@jonathanpeppers asked:

Do we know if it will be Android 12? If we don't know yet, we can leave this one alone:

The current plan will be to cherry-pick all the API-31 -related changes to the d16-11 branch. d16-11 will becomes Xamarin.Android 12.0, and main will become 12.0.99.

[jonpryor]

[Mono.Android] API-31 stabilization (#6174)

Context: https://developer.android.com/about/versions/12

API-31 has been enumified (88438081, cf2a39b2, cf2a39b2).
Time to declare it stable!

Set API-S as API-31, `$(TargetFrameworkVersion)`=v12.0,
`$(AndroidPackVersion)`=31.0.100, and declare as stable.

Update unit test files so `$(TargetFrameworkVersion)`= v12.0 (API-31)
*or* `$(TargetFrameworkVersion)` isn't needed, a'la 8140991e.

Rename `src/Mono.Android/Profiles/api-S.params.txt` to
`api-31.params.txt`, to match the stable API-level ID.

Updated `tests/api-compatibility/acceptable-breakages-vReference.txt`
because the default API compare will now be against `API-31`, not
`API-30`, and we deliberately removed the `[Category]` custom attribute
(e5a4beef), remove `[DataContractAttribute]` (101fea2c), and fix the
value of `Android.OS.BuildVersionCodes.R` (1b1ec097).

Support `$(AndroidUseLatestPlatformSdk)` in binding projects: once
API-31 is stable, binding projects with
`$(TargetFrameworkVersion)`=v11.0 and
`$(AndroidUseLatestPlatformSdk)`=true fail to build with:

	Task ResolveNuGetPackageAssets
	...
	Microsoft.NuGet.targets(198,5): Your project does not reference "MonoAndroid,Version=v12.0" framework. Add a reference to "MonoAndroid,Version=v12.0" in the "TargetFrameworks" property of your project file and then re-run NuGet restore.

What happened is that `$(TargetFrameworkVersion)` was changed to 12.0
during `Build`, but not during `Restore`. Thus NuGet restored with
`MonoAndroid,Version=v11.0` instead of `MonoAndroid,Version=v12.0`.

I found that we have this setup for other project types, but not
binding projects:

	<!--
	  NOTE:
	  This target runs during Restore, and there is no $(RestoreDependsOn) property.
	  There appears to be no other way to do this other than use BeforeTargets.
	-->
	<Target Name="_SetLatestTargetFrameworkVersionForPackageReference"
	    Condition=" '$(AndroidUseLatestPlatformSdk)' == 'True' "
	    BeforeTargets="_GetRestoreTargetFrameworksOutput"
	    DependsOnTargets="_SetLatestTargetFrameworkVersion">
	</Target>

If we move this target to `Xamarin.Android.Legacy.targets`, it will be
imported by binding projects.

We also get the added benefit of it not being in .NET 6.
`$(AndroidUseLatestPlatformSdk)` is not supported in .NET 6, so we
should not have this MSBuild target there.

Co-authored-by: Jonathan Peppers <jonathan.peppers@microsoft.com>