[dotnet] default $(SuppressTrimAnalysisWarnings) to true

Fixes: dotnet#10887

After bumping to .NET 6 Preview 3, we get hundreds of ILLINK warnings.
This is because of:

dotnet/sdk#16327

We need to default `$(SuppressTrimAnalysisWarnings)` to ignore these
warnings for now.

We should continue to use `TrimMode=link`, as that is the behavior
Xamarin developers get today.

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -142,6 +142,7 @@
 		<StartupHookSupport Condition="'$(StartupHookSupport)' == ''">false</StartupHookSupport>
 		<UseSystemResourceKeys Condition="'$(UseSystemResourceKeys)' == ''">true</UseSystemResourceKeys>
 		<UseNativeHttpHandler Condition="'$(_PlatformName)' != 'macOS' And '$(UseNativeHttpHandler)' == ''">true</UseNativeHttpHandler>
+		<SuppressTrimAnalysisWarnings Condition=" '$(SuppressTrimAnalysisWarnings)' == '' ">true</SuppressTrimAnalysisWarnings>
 	</PropertyGroup>
 
 	<PropertyGroup>