chore: Upgrade from .NET Core 2.1 to .NET Core 3.1 (#1112)

[akunzai]

Fixes #1112

• Upgrade from .NET Core 2.1 to .NET Core 3.1
• Migrate from Newtonsoft.Json to System.Text.Json for example projects that running on .NET Core 3.1.

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] I have updated my branch with the main branch
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have added the necessary documentation about the functionality in the appropriate .md file
• [x] I have added inline documentation to the code I modified

If you have questions, please file a support ticket, or create a GitHub Issue in this repository.

[thinkingserious]

Hello @akunzai!

Thank you for the PR!!

Reference: https://devblogs.microsoft.com/dotnet/net-core-2-1-will-reach-end-of-support-on-august-21-2021

This issue has been added to our internal backlog to be prioritized. Pull requests and +1s on the issue summary will help it move up the backlog.

With best regards,

Elmer

Internal Tracking: DI-1564

[eshanholtz]

@akunzai

A quick confirmation: These changes should not break users who choose to use this helper library on .NET Code 2.1, but rather upgrades our examples to use .NET Code 3.1?

[akunzai]

@eshanholtz

Yes,

1. In SendGrid.csproj, the TargetFrameworks was not modified, it keeps the compatibility with .NET Core 2.1.
2. In RetryDelegatingHandler.cs changes, the TaskCanceledException was inherited from OperationCanceledException, it keeps the compatibility too.
3. I upgrade tests to use .NET Core 3.1 too, not just examples.

[shwetha-manvinkurke]

Internal reference PR build

[shwetha-manvinkurke]

@akunzai thank you for the PR and for keeping it up to date! This is still in the backlog waiting to be prioritized.

[waynebrantley]

3.1 is no longer supported in dec 2022. I agree with your work, but perhaps just multi-target netstandard2.0 and net6.0. And does sendgrid/twilio have any intent on merging this?

[akunzai]

Hi @waynebrantley, thanks for your recognition, perhaps your can open another PR to remove outdated TFMs(netstandard1.3;net452;net40).

Please give this PR more thumbs up to make sendgrid/twilio raise priority of this PR.

[waynebrantley]

I would but only if we knew they were willing to merge it!
@mortenbock @rakatyal any thoughts?

[rakatyal]

Hi @waynebrantley and @akunzai! We definitely appreciate your contributions and will work on merging the changes on our end. I will appreciate if you can tag me once this is ready so that it doesn't get lost in my email. Thank you!

[akunzai]

Hi @rakatyal @eshanholtz @thinkingserious,

It is ready to merge for a long while. Please consider merge it before .NET Core 3.1 goes EOL, thanks.

[rakatyal]

Hi! Based on internal discussions, we won't be considering these changes since we will look to upgrade to .Net 6 taking into account the .Net Core EOL. Apologies for the confusion.

[akunzai]

Hi @rakatyal,

It's OK! never mind, I'm going to close this PR, thanks for your confirmation!

[Sharparam]

@rakatyal Seeing how the issues page has been disabled(?) I'll ask here: How are we supposed to report bugs/feature requests on this library now? The links in the README for "feature request" and "bug report" now point to a non-existing header in the CONTRIBUTING document.

[rakatyal]

Hi @Sharparam, you can report bugs/feature requests to us through Twilio SendGrid Support Help Center.