Target .NET 8

[weichch]

.NET 8 has been released and System.Json.Text has changed its targeting frameworks. Though the current targeting framework net6.0 and netstandard2.x would still work, it would be good to exam over any breaking changes and change the targeting framework accordingly.

We need to:

• Go over breaking changes in the System.Text.Json package of .NET 7
• Change targeting framework accordingly
• Target test projects to .NET 8
• Target benchmark project to .NET 8
• Update Github Actions to build and test in .NET 8
• Remove .NET 5 from all projects

[weichch]

Completed in #43