Mark macOS share extension as 'beta'

### Steps to Reproduce

1. Add the Share extension to Xamarin.Mac project (preferably to one that is intended for publishing in the App Store as there are even more problems in that case)
2. Try with various Visual Studio builds in the last 2 years
3. See how it works

### Expected Behavior
It would work properly always, at the very list if it is completely broken it would be urgently fixed

### Actual Behavior
Currently it doesn't work for months and this is not the first time. I understand that you may not have resources to fix this to work properly, but then at least be honest and professional and mark this as 'beta' (though currently it doesn't deserve even that label as one would expect at least some level of stability in that case).

Even worse, my issue for this hasn't been read for nearly two weeks: https://github.com/xamarin/xamarin-macios/issues/10911

While I would prefer that you actually make this work, at this point I think it is better to be honest and stop Xamarin.Mac developers from using this for anything but an experimental software.

### Environment

```
=== Visual Studio Community 2019 for Mac ===

Version 8.9.1 (build 34)
Installation UUID: f558466e-e717-453b-99e2-e900e934b179
GTK+ 2.24.23 (Raleigh theme)
Xamarin.Mac 6.18.0.23 (d16-6 / 088c73638)

Package version: 612000122

=== Mono Framework MDK ===

Runtime:
Mono 6.12.0.122 (2020-02/c621c35ffa0) (64-bit)
Package version: 612000122

=== Xamarin Designer ===

Version: 16.9.0.316
Hash: bd2705417
Branch: remotes/origin/d16-9
Build date: 2021-02-24 00:16:08 UTC

=== Roslyn (Language Service) ===

3.9.0-6.21152.10+c10f884b30737542ddd84ca889a4aad9281ce210

=== NuGet ===

Version: 5.8.0.6860

=== .NET Core SDK ===

SDK: /usr/local/share/dotnet/sdk/5.0.201/Sdks
SDK Versions:
5.0.201
5.0.103
5.0.102
5.0.101
5.0.100
3.1.407
3.1.406
3.1.405
3.1.404
3.1.403
3.1.402
3.1.401
3.1.302
3.1.301
3.1.300
3.1.202
3.1.200
3.1.102
3.1.101
3.1.100
3.0.101
3.0.100
2.1.701
2.1.700
MSBuild SDKs: /Applications/Visual Studio.app/Contents/Resources/lib/monodevelop/bin/MSBuild/Current/bin/Sdks

=== .NET Core Runtime ===

Runtime: /usr/local/share/dotnet/dotnet
Runtime Versions:
5.0.4
5.0.3
5.0.2
5.0.1
5.0.0
3.1.13
3.1.12
3.1.11
3.1.10
3.1.9
3.1.8
3.1.7
3.1.6
3.1.5
3.1.4
3.1.2
3.1.1
3.1.0
3.0.1
3.0.0
2.1.23
2.1.22
2.1.21
2.1.20
2.1.19
2.1.18
2.1.17
2.1.16
2.1.15
2.1.14
2.1.13
2.1.12
2.1.11

=== .NET Core 3.1 SDK ===

SDK: 3.1.407

=== Xamarin.Profiler ===

Version: 1.6.12.26
Location: /Applications/Xamarin Profiler.app/Contents/MacOS/Xamarin Profiler

=== Updater ===

Version: 11

=== Apple Developer Tools ===

Xcode 12.4 (17801)
Build 12D4e

=== Xamarin.Mac ===

Version: 7.8.2.5 (Visual Studio Community)
Hash: 3836759d4
Branch: d16-9
Build date: 2021-02-10 17:56:43-0500

=== Xamarin.iOS ===

Version: 14.14.2.5 (Visual Studio Community)
Hash: 3836759d4
Branch: d16-9
Build date: 2021-02-10 17:56:44-0500

=== Xamarin.Android ===

Version: 11.2.0.21 (Visual Studio Community)
Commit: xamarin-android/d16-9/93eab59
Android SDK: /Users/ivanicin/Library/Developer/Xamarin/android-sdk-macosx
Supported Android versions:
None installed

SDK Tools Version: 26.1.1
SDK Platform Tools Version: 28.0.2
SDK Build Tools Version: 28.0.3

Build Information:
Mono: 5e9cb6d
Java.Interop: xamarin/java.interop/d16-9@d6d86b2
ProGuard: Guardsquare/proguard/v7.0.1@912d149
SQLite: xamarin/sqlite/3.34.1@daff8f4
Xamarin.Android Tools: xamarin/xamarin-android-tools/d16-9@9d8924d

=== Microsoft OpenJDK for Mobile ===

Java SDK: /Users/ivanicin/Library/Developer/Xamarin/jdk/microsoft_dist_openjdk_1.8.0.25
1.8.0-25
Android Designer EPL code available here:
https://github.com/xamarin/AndroidDesigner.EPL

=== Android SDK Manager ===

Version: 16.9.0.22
Hash: a391de2
Branch: remotes/origin/d16-9
Build date: 2021-02-18 03:14:56 UTC

=== Android Device Manager ===

Version: 16.9.0.17
Hash: fc2b3db
Branch: remotes/origin/d16-9
Build date: 2021-02-18 03:15:18 UTC

=== Build Information ===

Release ID: 809010034
Git revision: 33bce2b7f3d45d05865cd3282f1c2cdc3a112aa0
Build date: 2021-03-04 11:21:38-05
Build branch: release-8.9
Xamarin extensions: 33bce2b7f3d45d05865cd3282f1c2cdc3a112aa0

=== Operating System ===

Mac OS X 10.16.0
Darwin 20.3.0 Darwin Kernel Version 20.3.0
Thu Jan 21 00:07:06 PST 2021
root:xnu-7195.81.3~1/RELEASE_X86_64 x86_64
```

### Build Logs

### Example Project (If Possible)