Fix broken links (#17786)

Samples links for the range >= aspnetcore3-0 are not working anymore.
I updated it with links of ASP.NET Core 3.1 because it is the latest LTS release available.
(Hint: the ASP.NET Core 3.0 samples are available here (https://github.com/dotnet/AspNetCore/tree/3.0/src/Security/samples). You can remove  "release/" from the paths and it will work. )

Author: MohamedSahbi

aspnetcore/security/authentication/samples.md

@@ -24,7 +24,7 @@ The [ASP.NET Core repository](https://github.com/dotnet/AspNetCore) contains the
 
 ## Run the samples
 
-* Select a [branch](https://github.com/dotnet/AspNetCore). For example, `Tag:v3.0.0`
+* Select a [branch](https://github.com/dotnet/AspNetCore). For example, `release/3.1`
 * Clone or download the [ASP.NET Core repository](https://github.com/dotnet/AspNetCore).
 * Verify you have installed the [.NET Core SDK](https://dotnet.microsoft.com/download/dotnet-core) version matching the clone of the ASP.NET Core repository.
 * Navigate to a sample in *AspNetCore/src/Security/samples* and run the sample with `dotnet run`.