Remove .NET Core 3 and .NET 5 targets

[Varorbc]

Why are unsupported target frameworks still supported? For example, netcoreapp3.0 and net5.0

[JamesNK]

Hasn't been a reason to yet. Might remove them when .NET 8 target is added.

[JamesNK]

Move to ActivitySource when old targets are dropped - open-telemetry/opentelemetry-dotnet#4695 (comment)

Add force yield in RetryCall and HedgeCall of GrpcCall.CallTask in .NET 8.

[Varorbc]

@JamesNK Can we think about dropping support for .NET 7.0, .NET Standard 2.0, and .NET Standard 2.1? Might we define the target frameworks in the following way instead

`<TargetFrameworks>$(NetCoreAppCurrent);$(NetCoreAppPrevious);$(NetCoreAppMinimum);$(NetFrameworkMinimum)</TargetFrameworks>`

[JamesNK]

.NET standard 2.0 is important. Maybe for the others.

[Varorbc]

What about .Net Standard 2.1?

[Varorbc]

I noticed that the precompile condition was not removed after the last update to the target framework.

grpc-dotnet/src/Grpc.AspNetCore.Server/GrpcEndpointRouteBuilderExtensions.cs

Line 40 in e9cc7e1

#if NET5_0_OR_GREATER

[JamesNK]

That condition is still needed to exclude code from .NET standard. No real reason to change it to a bigger version.