Bump Xamarin.AndroidX.Leanback from 1.0.0.16 to 1.0.0.21

Bumps [Xamarin.AndroidX.Leanback](https://github.com/xamarin/AndroidX) from 1.0.0.16 to 1.0.0.21.
- [Commits](https://github.com/xamarin/AndroidX/commits)

---
updated-dependencies:
- dependency-name: Xamarin.AndroidX.Leanback
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

ExoPlayer.Ext.Leanback/ExoPlayer.Ext.Leanback.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <AssemblyName>ExoPlayer.Ext.Leanback</AssemblyName>
     <RootNamespace>ExoPlayer.Ext.Leanback</RootNamespace>
@@ -7,7 +7,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Xamarin.AndroidX.Leanback" Version="1.0.0.16" />
+    <PackageReference Include="Xamarin.AndroidX.Leanback" Version="1.0.0.21" />
   </ItemGroup>
 
   <ItemGroup>