Drop support for .NET 4.5.2

[asosMikeGore]

If we dropped support for .NET 4.5.2 (now 3 years old) we can get faster feedback, compile times etc. The only issue is that servers would need to be upgraded to .NET 4.6.1.

Would be great to get some feedback and we can drop this into a major change along with new features.

[DaveAurionix]

.NET 4.6.1 is fine for us (or .NET 4.7).

[DaveAurionix]

@asosMikeGore do we still need support for anything other than .NET Standard 1.6?

Quite a few things are going to be faster, simpler or both if we can just target .NET Standard.

If we time-boxed feedback on this issue to two weeks from now and then, in the absence of major objections, dropped explicit targeting for the classic .NET Framework and only supported .NET Standard 1.6 would that be a positive step and help keep this project simple?

[asosMikeGore]

Had confirmation recently that our internal clients don't mind us dropping .NET Framework support. I've actually been chipping away (slowly) at this on the branch here:
https://github.com/ASOS/SimpleEventStore/tree/containerBuildV2

We recently had a few PRs to address issues on Linux so I'm migrating the build process to run under Linux containers & dropping Cake. There's a few minor bits of tidy up I want to do and then I'll get this integrated with the CI/CD server.