feat(csharp): upgrade to netcoreapp3.1

[RomainMuller]

From netcoreap3.0, in order to reflect the current
recommended framework version for .NET 3.x.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Commit Message

feat(csharp): upgrade to netcoreapp3.1 (#1241)

As netcoreapp3.0 is going to be EOL pretty soon (early March 2020),
upgrading our target framework to netcoreapp3.1 early in order to
catch any surprises. Since netcoreapp3.1 is nothing but an LTS version
of netcoreapp3.0 (plus some bug fixes), it should be transparent to users
who may only need to upgrade their .NET SDK.

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[rix0rrr]

What is the impact of this? Specifically, what are the benefits and what are the costs?

• Are people using .NET core 3.1 currently unable to use jsii because we're targeting 3.0?
• Do people using .NET core 3 now have to upgrade to use jsii after this change?
• What are the advantages of upping our requirements?
• Does .NET core 3.1 have new features we are planning to take advantage of?
• Is .NET core 3.0 end of life?

To me, it seems nice to our customers to have as broad compatibility as possible for our software, to not force them to upgrade unnecessarily. It's why we target Java 8 (instead of 13) and NodeJS 10 (instead of 13) and Python 3.6 (instead of 3.7).

[RomainMuller]

@rix0rrr

• netcoreapp3.0 can be built by .NET Core 3.1 SDKs
• Yes, people who have not yet upgraded to the current release of the dotnet-sdk will need to do so
• There is no advantage beyond the fact that netcoreapp3.0 is soon to become EOL.
• There's no new features we plan on using
• netcoreapp3.0is going EOL on March 3, 2020. I'd rather upgrade early and be able to revert while we work out wrinkles than do it after EOL. netcoreapp3.1 is the LTS release.
  • The upgrade was recommended by our .NET SDK engineers, who strongly advised we upgrade our target framework to 3.1 on the first occasion (it was not released yet when we moved to netcoreapp3.0).

Ref: https://dotnet.microsoft.com/platform/support/policy/dotnet-core

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[rix0rrr]

Thanks for the info!

[RomainMuller]

(Note that this code requires a new Superchain image to be live before it'll pass PR validations)

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