Update unobserved task exception integration

[mattjohnsonpint]

We had a few extraneous null checks in the TaskUnobservedTaskExceptionIntegration.

• From Should UnobservedTaskExceptionEventArgs.Exception be nullable? dotnet/runtime#32454 it appears that the exception will never be null.
• The _hub variable will never be null within Handle, because that can only occur after registering the integration.

Also, we have been calling this TaskUnobservedTaskExceptionIntegration with the word Task twice. The exception raised is called UnobservedTaskException, so the leading word Task is redundant. Renaming all usage to UnobservedTaskExceptionIntegration.

This rename makes one public change. The DisableTaskUnobservedTaskExceptionCapture extension method is now obsolete in favor of DisableUnobservedTaskExceptionCapture.

[bitsandfoxes]

That's a nice little cleanup!