Disable workload for projects targeting <net6.0 (#59)

eng/nuget/Microsoft.NET.Workload.Emscripten.Manifest/WorkloadManifest.targets

@@ -1,9 +1,23 @@
 <Project>
   <PropertyGroup Condition="'$(RuntimeIdentifier)' == 'browser-wasm'">
+    <BrowserWorkloadDisabled Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp' and !$([MSBuild]::VersionGreaterThanOrEquals('$(TargetFrameworkVersion)', '6.0'))">true</BrowserWorkloadDisabled>
+  </PropertyGroup>
+
+  <PropertyGroup Condition="'$(RuntimeIdentifier)' == 'browser-wasm' and '$(BrowserWorkloadDisabled)' == 'true'">
+    <_NativeBuildNeeded Condition="'$(RunAOTCompilation)' == 'true'">true</_NativeBuildNeeded>
+    <WorkloadDisabledWithReason Condition="'$(_NativeBuildNeeded)' == 'true'">WebAssembly workloads (required for AOT) are only supported for projects targeting net6.0+</WorkloadDisabledWithReason>
+  </PropertyGroup>
+
+  <PropertyGroup Condition="'$(RuntimeIdentifier)' == 'browser-wasm' and '$(BrowserWorkloadDisabled)' != 'true'">
     <UsingBrowserRuntimeWorkload Condition="'$(RunAOTCompilation)' == 'true' or '$(UsingMicrosoftNETSdkBlazorWebAssembly)' != 'true'" >true</UsingBrowserRuntimeWorkload>
     <UsingBrowserRuntimeWorkload Condition="'$(UsingBrowserRuntimeWorkload)' == ''" >$(WasmNativeWorkload)</UsingBrowserRuntimeWorkload>
   </PropertyGroup>
 
+  <PropertyGroup Condition="'$(BrowserWorkloadDisabled)' == 'true'">
+    <UsingBrowserRuntimeWorkload>false</UsingBrowserRuntimeWorkload>
+    <WasmNativeWorkload>false</WasmNativeWorkload>
+  </PropertyGroup>
+
   <ImportGroup Condition="'$(RuntimeIdentifier)' == 'browser-wasm' and '$(UsingBrowserRuntimeWorkload)' == 'true'">
     <Import Project="Sdk.props" Sdk="Microsoft.NET.Runtime.Emscripten.Python" Condition="!$([MSBuild]::IsOsPlatform('Linux'))" />
     <Import Project="Sdk.props" Sdk="Microsoft.NET.Runtime.Emscripten.Node" />