.NET 4.5.2 reaches EOL on 26 April 2022

[Deadpikle]

https://devblogs.microsoft.com/dotnet/dotnet-framework-45-46-461-end-of-support/

This should be handled at next version bump (3.x). We will drop support for .NET 4.5.2 and bump up to 4.6.2 at the minimum. That might allow us to make other upgrades, such as using System.Text.Json for everything.