Set a default for the Http3Support feature switch

Control over this is moving from dotnet/runtime repo build time to publish time so we need to set a default that matches the old build-time setting.

Ref #117012
Ref dotnet/sdk#49564

Author: MichalStrehovsky

src/mono/nuget/Microsoft.NET.Workload.Mono.Toolchain.Current.Manifest/WasmFeatures.props

@@ -7,6 +7,7 @@
     <UseSizeOptimizedLinq Condition="'$(UseSizeOptimizedLinq)' == ''">true</UseSizeOptimizedLinq>
     <UseSystemResourceKeys Condition="'$(UseSystemResourceKeys)' == ''">true</UseSystemResourceKeys>
     <EnableUnsafeUTF7Encoding Condition="'$(EnableUnsafeUTF7Encoding)' == ''">false</EnableUnsafeUTF7Encoding>
+    <Http3Support Condition="'$(Http3Support)' == ''">false</Http3Support>
     <HttpActivityPropagationSupport Condition="'$(HttpActivityPropagationSupport)' == ''">false</HttpActivityPropagationSupport>
     <DebuggerSupport Condition="'$(DebuggerSupport)' == '' and '$(Configuration)' != 'Debug'">false</DebuggerSupport>
     <WasmEnableStreamingResponse Condition="'$(WasmEnableStreamingResponse)' == ''">true</WasmEnableStreamingResponse>