chore: Make it an error

(cherry picked from commit b582eea82c)

src/Uno.Wasm.Bootstrap/build/Uno.Wasm.Bootstrap.targets

@@ -73,7 +73,7 @@
 
 	<PropertyGroup Condition="
 			'$(TargetFrameworkIdentifier)'=='.NETCoreApp'
-			and $([MSBuild]::VersionGreaterThanOrEquals($(NETCoreSdkVersion), '9.0'))">
+			AND $([MSBuild]::VersionGreaterThanOrEquals($(NETCoreSdkVersion), '9.0'))">
 
 		<!-- .NET 9 SDK Compatibility -->
 		<EnableDefaultCompressedItems>false</EnableDefaultCompressedItems>
@@ -199,8 +199,8 @@
 			  Condition="'%(Extension)' == '.dll'" />
 		</ItemGroup>
 
-		<Warning
-			Condition="$([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '9.0'))"
+		<Error
+			Condition="$([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '9.0')) AND '$(UnoDisableNet9Validation)' != 'true' "
 			Text="This version of the Uno.Wasm.Bootstrap package does not support the net9.0 target framework and later, starting from the .NET 9.0 SDK Preview 7. The net8.0 TargetFramework is supported." />
 
 		<PropertyGroup>