Target .NET Standard 2.0 / .NET 8 / .NET 10

[eXpl0it3r]

Newer .NET versions provide a lot of performance and general language improvements.

We should be able to retain .NET Framework compatibility by continuing to support .NET Standard 2.0 for the Core library.
For WPF and WinForms the compatibility needs to be broken in order to support the newer .NET versions (afaik).

But to benefit from all the improvements one should multi-target those libraries that support the .NET Standard 2.0.

• HtmlRenderer.Core: netstandard2.0 / net8.0 / net10.0
• HtmlRenderer.PdfSharp: netstandard2.0 / net8.0 / net10.0
• HtmlRenderer.WinForms: net8.0-windows / net10.0-windows
• HtmlRenderer.WPF: net8.0-windows / net10.0-windows
• HtmlRenderer.Mono: Deprecated

Additional related updates

• [ ] Update PDFsharp: #221
• [ ] Use System.Text.Json instead of custom/hacky IndexOf JSON parsing