7.0: update target framework from net461 to netcoreapp3.1

[bollhals]

Proposed Changes

7.0 Change

• Deprectates the net461 branch and activates netcoreapp3.1
• Cleans up NetworkOrder(De)Serialization class and aligns read/write for double

Types of Changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• All tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in related repositories

[danielmarbach]

I would not set the language version to latest. Because that allows you to use language features that might not be supported by the TFMs chosen here. If you leave it blank you get automatically the proper language version supported by the TFM intersection

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/configure-language-version

[bollhals]

Doesn't "The compiler accepts syntax from the latest released version of the compiler" mean it will always choose the latest possible whatever framework version we selected?

[bording]

No, it will choose the the latest version the compiler knows about, overriding the version that would have been picked by the compiler based on the TFM.

@danielmarbach is right that it should not be set to get the automatic behavior that we want here.

[bollhals]

Well that's wrong then, sorry. I'll update it in one of my next PRs. Thanks for the review

[michaelklishin]

@bollhals I failed to backport this to 7.x twice because there are too many optimizations and [the elimination of] code generation changes in master. Can you please submit a version of this based on 7.x?