[maccatalyst] Make sure MacProxy is included in System.Net.Http (#91599)

#90258 identified that `HttpClient.DefaultProxy` returned `System.Net.Http.HttpNoProxy` when using MacCatalyst. This is due to a condition in System.Net.Http.csproj not including maccatalyst as a condition and as a result, skipped adding the MacProxy sources.

Way back when, #47823 had the condition, but this appears to have changed in #48652 when we thought MacCatalyst would inherit settings from iOS. That turned out to not be the case and this check was never added back.

Co-authored-by: Steve Pfister <steve.pfister@microsoft.com>

src/libraries/System.Net.Http/src/System.Net.Http.csproj

@@ -316,11 +316,11 @@
     <Compile Include="System\Net\Http\SocketsHttpHandler\HttpEnvironmentProxy.Unix.cs" />
     <Compile Include="System\Net\Http\SocketsHttpHandler\CurrentUserIdentityProvider.Unix.cs" />
   </ItemGroup>
-  <ItemGroup Condition="'$(TargetPlatformIdentifier)' != '' and '$(TargetPlatformIdentifier)' != 'windows' and '$(TargetPlatformIdentifier)' != 'browser' and '$(TargetPlatformIdentifier)' != 'osx' and '$(TargetPlatformIdentifier)' != 'ios' and '$(TargetPlatformIdentifier)' != 'tvos'">
+  <ItemGroup Condition="'$(TargetPlatformIdentifier)' != '' and '$(TargetPlatformIdentifier)' != 'windows' and '$(TargetPlatformIdentifier)' != 'browser' and '$(TargetPlatformIdentifier)' != 'osx' and '$(TargetPlatformIdentifier)' != 'ios' and '$(TargetPlatformIdentifier)' != 'tvos' and '$(TargetPlatformIdentifier)' != 'maccatalyst'">
     <Compile Include="System\Net\Http\SocketsHttpHandler\HttpNoProxy.cs" />
     <Compile Include="System\Net\Http\SocketsHttpHandler\SystemProxyInfo.Unix.cs" />
   </ItemGroup>
-  <ItemGroup Condition="'$(TargetPlatformIdentifier)' == 'osx' or '$(TargetPlatformIdentifier)' == 'ios' or '$(TargetPlatformIdentifier)' == 'tvos'">
+  <ItemGroup Condition="'$(TargetPlatformIdentifier)' == 'osx' or '$(TargetPlatformIdentifier)' == 'ios' or '$(TargetPlatformIdentifier)' == 'tvos' or '$(TargetPlatformIdentifier)' == 'maccatalyst'">
     <Compile Include="System\Net\Http\SocketsHttpHandler\SystemProxyInfo.OSX.cs" />
     <Compile Include="System\Net\Http\SocketsHttpHandler\MacProxy.cs" />
     <Compile Include="$(CommonPath)Interop\OSX\Interop.CoreFoundation.cs"