Update target frameworks

[Joy-less]

Changes the target frameworks from (netstandard1.3, netstandard2.1) to (netstandard2.0, netstandard2.1, net8.0, net9.0).

Targeting netstandard1.3 is no longer recommended by Microsoft since the frameworks compatible with netstandard1.3 but not by netstandard2.0 are long out-of-support.

Targeting net8.0 and net9.0 should give performance benefits.

Also updates the test project packages.

[mqudsi]

Thanks for the initiative, but the target frameworks have been carefully chosen.

netstandard1.3 is the newest target that is supported for UAP apps (deprecated but still in use by many). I'm not aware of any benefits in targeting net8.0, net9.0 when the code doesn't take advantage of any of the apis exclusive to those targets; the netstandard2.1 target would be compiled and uploaded to nuget using the latest version of the sdk and should see the same codegen benefits that a net9.0 target would, if I'm not mistaken.