[ASP.NET Core] .NET7+ ActivitySource selection logic

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Today if we detect .NET 7 or greater we resolve the ActivitySource to listen to using the IServiceProvider. This is to mirror what ASP.NET Core does under the hood. See: AddAspNetCoreInstrumentationSources.

@noahfalk Left a comment suggesting that we just listen to the default source by name.

Opening this issue to track the discussion further.