[StaticWebAssets] Switch to source generated System.Text.Json

[javiercn]

~1s win total.
Before

Build succeeded in 12.8s
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  mvc-dotnet-9 succeeded (11.8s) → bin\Debug\net9.0\mvc-dotnet-9.dll

Build succeeded in 12.7s
You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
  mvc-dotnet-9 succeeded (11.6s) → bin\Debug\net9.0\mvc-dotnet-9.dll

Build succeeded in 12.6s
You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
  mvc-dotnet-9 succeeded (11.7s) → bin\Debug\net9.0\mvc-dotnet-9.dll

Build succeeded in 12.6s

After

Build succeeded in 12.2s
You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
  mvc-dotnet-9 succeeded (10.9s) → bin\Debug\net9.0\mvc-dotnet-9.dll

Build succeeded in 11.9s
You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
  mvc-dotnet-9 succeeded (10.4s) → bin\Debug\net9.0\mvc-dotnet-9.dll

Build succeeded in 11.4s
You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
  mvc-dotnet-9 succeeded (10.8s) → bin\Debug\net9.0\mvc-dotnet-9.dll

Build succeeded in 11.7s

[MackinnonBuck]

Nice!