Target System.Text.Json 5.0.0 final

[bartelink]

Update FsCodec.SystemTextJson to use 5.0.0 final
Updated build and projects to rely on 5.0.200 SDK (local build works on my Mac from command line and Rider with only that)

[bartelink]

@klimisa I have no idea what I did, but it seems this works in latest Rider on a Mac with only the 5.0 SDK installed
I've killed an old branch I had; Probably best to close #56 now ? (if you have time, would be good to check it's all happy of course!)

[klimisa]

Let me have a look now!

PS: sorry for not being active, it have been a busy and it's still is.

[klimisa]

All works fine on Linux!!!

Rider :

Shell:

cc/ @bartelink

[bartelink]

Thanks for validating @klimisa
Will merge now.

I'm thinking I'll release it as V 2.2.1, though arguably it merits a bigger increment. I guess it definitely will when I get bored of looking at #59 :D