use .NET Standard 2.0 and later as only build target

[Aaronontheweb]

close #3851 - dropping .NET Framework 4.5.2 support.

Draft PR because we need to get rid of the old Helios transport package first.

[Aaronontheweb]

#3945 needs to be merged into dev first before this can run successfully.

[Aaronontheweb]

Welp, here's where this build hit the wall:

https://github.com/akkadotnet/akka.net/blob/0a8c0390ee24927e3c381a99bca079ebc7e5b8e9/src/core/Akka/Configuration/Hocon/AkkaConfigurationSection.cs

... @Arkatufus - so what should we do with this given that we're moving HOCON as a stand-alone feature into Akka.NET? If we get rid of AkkaConfigurationSection?

[Aaronontheweb]

I think what we're going to end up doing here, so as to not introduce too many behavior changes at once, is wait and see how the new stand-alone HOCON integration goes and see if that library can offer a good solution.

[Arkatufus]

Welp, here's where this build hit the wall:

https://github.com/akkadotnet/akka.net/blob/0a8c0390ee24927e3c381a99bca079ebc7e5b8e9/src/core/Akka/Configuration/Hocon/AkkaConfigurationSection.cs

... @Arkatufus - so what should we do with this given that we're moving HOCON as a stand-alone feature into Akka.NET? If we get rid of AkkaConfigurationSection?

This should fix it: Arkatufus@3e08c21

[Arkatufus]

We should mark the old System.Configuration implementation as deprecated to prepare for standalone HOCON integration.

[Aaronontheweb]

We should mark the old System.Configuration implementation as deprecated to prepare for standalone HOCON integration.

Have some API ideas for how we can do this...

[Aaronontheweb]

@Arkatufus retrying this PR with your fix

[Aaronontheweb]

Looks like that created some compilation problems on Linux - the same ValueTuple stuff we ran into in some other projects. System.Configuration.ConfigurationManager might take a dependency on something that references a different version of this same library.

[Aaronontheweb]

Looks like we're good to go here. Going to merge this in and set the groundwork for stand-alone HOCON.