Fix non-official and outdated GitHub repository links

@thangchung and @tibitoth, I reviewed the repository links and found a few that should be corrected.

Non-official or misleading repositories

• Refit
  • Current link: https://github.com/paulcbetts/refit
  • Official repository: https://github.com/reactiveui/refit The old namespace has been taken over and no longer contains the Refit project. This one should be fixed as a priority.
• Stripe
  • Current link: https://github.com/ServiceStack/Stripe
  • Official Stripe SDK: https://github.com/stripe/stripe-dotnet ServiceStack/Stripe is a third-party client, not Stripe’s official .NET SDK. It should either be replaced or explicitly described as a ServiceStack client.
• SendGrid Client
  • Current link: https://github.com/0xdeafcafe/sendgrid-dotnet
  • Official SDK: https://github.com/sendgrid/sendgrid-csharp The official SDK is already listed elsewhere, so the third-party entry could simply be removed.

Outdated links that redirect to official repositories

The following links still redirect correctly, but should be replaced with their canonical URLs:

• Microsoft/aspnet-api-versioning → dotnet/aspnet-api-versioning
• JeremySkinner/FluentValidation → FluentValidation/FluentValidation
• OfficeDev/Open-XML-SDK → dotnet/Open-XML-SDK
• jbogard/MediatR → LuckyPennySoftware/MediatR
• chkr1011/MQTTnet → dotnet/MQTTnet
• tonerdo/coverlet → coverlet-coverage/coverlet
• kblok/puppeteer-sharp → hardkoded/puppeteer-sharp
• aspnet/EntityFramework → dotnet/efcore

There are also many legacy aspnet/, Microsoft/, and OfficeDev/* links throughout the document. Most are old official locations rather than non-official repositories, but updating them would avoid relying on GitHub redirects.

- https://github.com/Microsoft/aspnet-api-versioning
+ https://github.com/dotnet/aspnet-api-versioning

- https://github.com/JeremySkinner/FluentValidation
+ https://github.com/FluentValidation/FluentValidation

- https://github.com/OfficeDev/Open-XML-SDK
+ https://github.com/dotnet/Open-XML-SDK

- https://github.com/jbogard/MediatR
+ https://github.com/LuckyPennySoftware/MediatR

- https://github.com/chkr1011/MQTTnet
+ https://github.com/dotnet/MQTTnet

- https://github.com/tonerdo/coverlet
+ https://github.com/coverlet-coverage/coverlet

- https://github.com/kblok/puppeteer-sharp
+ https://github.com/hardkoded/puppeteer-sharp

- https://github.com/aspnet/EntityFramework
+ https://github.com/dotnet/efcore