Add Adjust Targeted Frameworks

[kenny-sellers]

Description

Add support for latest build in .NET 6 and update all .NET 5 builds to latest stable patch. Remove targeting of .net standard.

Tasks

Update all projects and sample code to compile with .NET 6
Update all projects and sample code to compile with latest patch in .NET 5
Update NuGet packages to ship .NET 5, .NET 6 and .NET 4.8
Add build to compile and test with .NET48, .NET5, and .NET 6
Update Benchmarks to run jobs for all frameworks. An example of this can be found here: An example of multi-targeting the jobs can be found here: https://github.com/FileOnQ/Imaging.Heif/blob/main/benchmarks/tools/Thumbnail.cs